Through email, phone calls, forums and occasional blog comments we read, we see questions that are often repeated so we maintain a FAQ to provide the public with these questions and our responses.

If you have a question or questions, you can email them to arne@northamericansportsman.com

When sending an email, please provide your full name, address and phone number.  We may choose to respond with a phone call.   However, without this information, we will not respond to messages.

Please do not post questions or send private messages on forums expecting a response.  We simply don’t have the time to monitor forums. 

Due to International Trade in Arms Regulations (ITAR), we cannot provide technical information to non-US Citizens or non-legal residents of the United States of America.


Question – Is the 375 RAPTOR just a 308 Winchester necked up to .375 Caliber?

Answer – No, the 375 RAPTOR changes the shoulder angle from 20 degrees to 35 degrees to provide a well defined datum point for the cartridge to headspace.  In addition, the 375 RAPTOR is .150″ shorter than the 308 Winchester which permits up to .935″ of bullet exposure while keeping within magazine length – required to load the 350 grain Sierra Match King magazine length. 

Question – What led you to decide on a .375 caliber in a modified 308 Winchester case for the 375 RAPTOR?

Answer – In conducting research on the qualities that come together to create an ideal hunting cartridge and rifle with many seasoned big game hunters experienced not only in taking the largest North American game animals, but in the pursuit of African game (We are talking people whose life passion is hunting and see a rifle as a tool).   There was a consensus of opinion that the ideal cartridge combines a large frontal area caliber with 250 to 300 grain bullet that impacts the game at a velocity between 1800 and 2400 fps at ranges between 50 and 300 yards and accomplishes this level of performance in a lightweight 18-22 inch barrel rifle that can be carried all day.  Heavier / longer rifles they felt were cumbersome in the field and high velocity cartridges did not work as effectively at the ranges common in hunting bear, moose, wild boar or African game.   Taking that input, from people who live to hunt and have done so for decades,  the 375 RAPTOR came into vision, but not only for performance reasons, but also to eliminate the proprietary collection of parts and parent brass that plagues other big bore offerings created for other lightweight rifles.

Question – Why was the 375 RAPTOR neck shortened and not just left full length after resizing? 

Answer – The 375 RAPTOR was designed to be able to shoot magazine length the full range of .375 caliber bullets including the Sierra 350-Grain Match King which has a .720 BC below 1700 fps.  Why does that matter?  If you shoot subsonic, the 350-Grain Match King has 60% more energy at 100 yards when fired subsonic than the 300 AAC BLACKOUT shooting a heavy OTM bullet subsonic.   In addition, the 375 RAPTOR gives the end-user options like the Barnes 350-Grain TSX or Lehigh 295-Grain Controlled Fracture.  

In the following photo, you can see how the Sierra 250-Grain Game King compares to the Sierra 350-Grain Match King. 

Sierra Bullet Comparison

Question – Does the 375 RAPTOR’s 35 degree shoulder impact feeding in a semi-auto rifle?

Answer – No, due to the large neck diameter of the 375 RAPTOR, the height of the body at the shoulder in relation to the neck diameter is only .024″ per side.  Increasing the shoulder angle to 35 degrees provided a well-defined datum point on what is a very minimal shoulder area for head-spacing.  

Question – How does the 375 RAPTOR compare to a 375 H&H MAGNUM?

Answer – While both are .375 caliber, the cartridges are in different classes.  The 375 H&H MAGNUM is formidable cartridge, but it requires a long magnum action and a long barrel to make use of the large case capacity.   In contrast, the 375 RAPTOR is a cartridge designed for short-action rifles and the 308 AR (AR-10, LR308, SR-25 and G2) and can be used with barrel lengths from 10.75 inches to 22 inches giving the user a light weight easy handling hunting rifle.  So the 375 RAPTOR and the 375 H&H MAGNUM are in different classes, designed for entirely different platforms. 

Question – The 300 AAC BLACKOUT has been shown to chamber in a 5.56×45 chamber with dangerous results. Can the 375 RAPTOR chamber in a 308 WINCHESTER barrel creating a dangerous situation? 

Answer – No, the 375 RAPTOR cannot chamber in a 308 Winchester barrel.  

A .375 caliber bullet has a major diameter that is .032″ larger than the outside neck diameter of a 308 Winchester round and the 375 RAPTOR case neck is  .062″ larger than the outside neck diameter of the 308 Winchester.   

As the expression goes, “a picture is worth a thousand words”, the following photo is a 308 Winchester barrel made by a well-known manufacturer with a 375 RAPTOR dummy round, loaded with a 270 Speer BTSP,  pushed as far into the chamber as possible.  As you can see, the 375 RAPTOR extends from the chamber a significant amount, so much that the base of the case extends beyond the rear face of the barrel extension.   As such, the bolt lugs will not engage the barrel extension giving a very clear visual indication to the user that something is wrong.  Furthermore, the bolt carrier will not be far enough rearward that the trigger hammer will not engage the firing pin. 


Now, if you are someone who accidentally chambers a .308 Winchester in a 375 RAPTOR, the 308 Winchester will chamber.  Upon firing, the case will fire-form with lower pressure than a .308 Winchester as the pressure vents around the bullet which is .067″ smaller in diameter.  The bullet will exit the bore without any spin since it will not engage the rifling.   The firing will be noticeably different compared to firing the 375 RAPTOR and the case may not eject due to the lack of proper gas pressure. 

Question – How is the 375 RAPTOR more powerful than other big bores for the AR-15 and AR-10, isn’t bigger better? 

Answer – The 375 RAPTOR power advantage comes from the vastly superior ballistic coefficients of bullets for a given weight.   For example, here are a sampling of different hunting bullets in different calibers:

375 Caliber – 300 Grain Sierra Game King – .470 G1 BC

452 Caliber – 300 Grain Speer Deep Curl- .233 G1 BC or 300 Grain Hornady XTP Mag .200 G1 BC

.458 Caliber – 300 Grain Speer JHP – .206 G1 BC or 300 Grain Hornady HP .197 BC

.499 Caliber – 300 Grain Speer Deep Curl – .155 G1 BC or 300 Grain Hornady XTP Mag .120 G1 BC

If all bullets are fired at equal velocities, the higher ballistic coefficient bullet (.375) will retain velocity and deliver higher terminal energy (foot pounds) on target.   Using that understanding, lets calculate the 200-yard terminal energy of each bullet fired at a muzzle velocity of 2,165 fps:

.375 Caliber – 300 Grain Sierra Game King @ 2,165 fps MV – 2,221 Foot Pounds @ 200 yards

.452 Caliber – 300 Grain Speer Deep Curl @ 2,165 fps MV – 1,470 Foot Pounds @ 200 yards

.458 Caliber – 300 Grain Speer JHP @ 2,165 fps MV – 1,432 Foot Pounds @ 200 yards

.499 Caliber – 300 Grain Speer Deep Curl @ 2,165 fps MV – 1,135 Foot Pounds @ 200 yards

The results are clearly evident, using a 300 grain bullet, .375 Caliber has 195% greater terminal energy at 200 yards than .499 Caliber; 155% greater terminal energy than .458 Caliber and 151% greater terminal energy than .452 Caliber.

Keep in mind that the .499 Caliber (50 Beowulf) and .458 Caliber (.458 SOCOM) options for the AR-15, cannot achieve 2,165 fps in a sub 20 inch barrel configuration with a 300 grain bullet and you should never attempt to push those cartridges beyond their design limitations.  In contrast, both of our big bore designs (45 RAPTOR and 375 RAPTOR) can achieve 2,165 fps (or higher velocity) when loaded in sub 20 inch barrel configurations. 

Question – When shooting an SBR (Short Barrel Rifle), what is the minimum velocity required to remain effective at 200 yards?

Answer – This brings up an important discussion with so many people ordering barrels from 10.75 inches to 22 inches.     To quantify being effective at 200 yards, lets use the 1800 fps on target velocity specified by bullet manufacturers to insure expansion in a hunting application.   The minimum muzzle velocity to result in a 1800 fps on target velocity at 200 yards is as follows:

  • 250 Sierra Game King : 2250 fps
  • 300 Sierra Game King : 2140 fps
  • 235 Speer : 2535 fps
  • 270 Speer BTSP : 2165 fps
  • 260 Nosler Accubond : 2125 fps
  • 260 Nosler Partition : 2310 fps
  • 300 Nosler Accubond : 2125 fps
  • 300 Nosler Partition : 2195 fps
  • 250 Barnes TSX BT : 2165 fps
  • 250 Hornady GMX : 2165 fps

Keep in mind these are minimum muzzle velocities required to achieve 1800 fps on target at 200 yards.   Any increase in muzzle velocity will result in an increase in maximum range where 1800 fps in achieved on target.    For example, the Nosler Accubond in both 260 and 300 grains with a muzzle velocity of 2300 fps will achieve a 300 yard range with the bullet impacting at 1800 fps.    Using an 18-22 inch barrel will achieve higher muzzle velocities than a 10.75 or 12 inch barrel and therefore, greater effective range.   This does not negate the value of a 10.75 or 12 inch barrel SBR especially when confronting a Grizzly Bear in the woods of Alaska.  

Question – What about a suppressed 375 RAPTOR using subsonic loads for hunting? 

Answer – The 375 RAPTOR was developed specifically to use the Sierra 350 grain Match King loaded from the magazine which is no small feat given the length of the bullet ogive.   In addition, it works magazine length with the Lehigh 295 Controlled Fracture bullet designed specifically for subsonic use.  Other possible subsonic bullets to consider would include the Sierra 300-Grain Game King or Barnes TSX Flat Base in either 300 or 350 grains.   Using the Sierra 350 grain Match King, the 375 RAPTOR has 60% more energy at 100 yards than the 300 AAC Blackout.   Suppressors tested include the Bowers VERS 458, VERS 50 and VERS 9S  equipped with stainless steel baffles.  The suggested powder for subsonic use is Vihtavouri Tin Star which is a bulky powder developed for cowboy action shooting.   Not only does this powder result in high case fill levels, it is very clean burning.

Question – What can you tell us about the 375 RAPTOR for tactical applications? 

Answer – Our public discussion on the 375 RAPTOR, or any of our others cartridges, is limited to sporting use, primarily the hunting of wild game or target shooting.    That being said, we don’t publicly discuss possible tactical use of any cartridge or platform.  If an organization has questions regarding any cartridge for professional use, they are welcome to contact us directly.  You can be assured, we will never discuss the existence of those conversations or any professional use, if it occurs, publicly.  

Question – If you were to recommend a few bullet for the 375 RAPTOR, what would they be? 

Answer – The 3 mainstream bullets I would suggest include the Speer 270-grain BTSP, the Sierra 250-grain Game King and the Nosler 260-grain Accubond.    For speciality bullet makers, the Lehigh Defense 175 grain Match or Controlled Fracture bullets is especially useful for SBR applications or light recoiling (for a .375) carbine.   

Question – What 375 RAPTOR barrel length do you recommend? 

Answer – In a lightweight carbine, an 18-inch barrel would be a good choice.  If you seek to extract more range, a 22 inch barrel would give you an extra 50-100 fps over an 18-inch barrel, depending on the powder / bullet combination.

Question – What is the recommended 375 RAPTOR gas system for the AR-10 / LR308, SR-25 and G2?

Answer – Different barrel lengths ranges have different gas system options

  • 10.75: Pistol or Carbine
  • 12.75-14.75: Carbine
  • 16 inch: Carbine
  • 18 inch: Carbine to Mid-Length
  • 20 inch: Mid Length to Rifle Length
  • 22 inch: Rifle Length

Gas port sizes change throughout the range and the dynamics especially change if you plan to use a suppressor and what suppressor you use.   For example, an 18-inch mid-length, is going to require a gas port between .104″ and .1065″ which is the same as a 10.75-inch barrel with a carbine length gas system.  In contrast, a 20-inch mid-length is going to require a gas port between .067″ and .070″ because you are adding 2-inches of barrel.  You can also see if you want a 16-inch barrel with a mid length, your gas port will be even larger than an 18-inch barrel

We do not believe in over gassing guns which seems to be a common problem when buying barrels because no barrel maker wants a barrel to come back because it is short cycling.  The problem of over gassing becomes even worse, when you add in the number the people who now own suppressors, shooting both subsonic and supersonic ammo and the options can give anyone a headache.

So here is what we do, regardless of AR-15 or AR-10 and cartridge:

  • Use a high quality adjustable gas block (that actually adjusts from wide open to fully closed) and use the maximum gas port size (.120″) and tune your gun to cycle reliably with your chosen load.  We actually designed a gas block we use that has a selection of jets (like carburetors had).  Not only do we know exactly what port we end up with, but can change jets at the range without even removing the gas block.  If you want one of these, call Scott Medesha at 480-986-5876 and they cost between $100 and $150 depending on how many jets you want.
  • If you absolutely don’t want an adjustable gas block (we have all had bad experiences with ones that leak or the adjustment seems like a knob you just turn and turn), you can start with a small port (you can always make the port larger) and increase the gas port by heading to your machinist / industrial supply and get a collection of high speed steel or carbide reamers.  I paid $14.95 each for individual carbide reamers as a walk-in customer at Bass Tool & Supply in Houston, TX.   To expand a gas port,  I use a barrel clamp (see photo), a small drill press, press vise and some light oil to lubricate the the reamer and increase the port in small steps (.004″-.006″).  Be sure to properly line up the reamer and the existing port, take your time and only open the existing port, not going too deep and hitting the other side of the barrel.  Of course, be sure to clean the barrel afterwards to remove an small shavings.  The whole process takes about 5 minutes.
Barrel Clamp

Wheeler Universal Barrel Clamp (Amazon.com)

If you want to know what it looks like when you have a proper gas port, your rifle will be throwing brass about 4-5 feet away, not 15 feet.   Keep in mind, when you are working up loads and are starting with light loads, you may experience short cycling or no cycling, that is normal as you are generating enough gas pressure to cycle the action.

Question – Can I buy 375 RAPTOR brass?

Answer – We thought about having native brass produced, but common sense ruled out producing special runs of brass that would end up being higher priced to the consumer compared to buying commercial 308 Winchester or 7.62 NATO brass and performing standard reloading operations of resizing and case trimming.   The 375 RAPTOR is just that easy to make.  The instructions for making 375 RAPTOR brass can be found on the loading tab of this website.  

Kaleo Arms in Hawaii has indicated that they will be able to offer 375 RAPTOR brass made from reconditioned Lake City Brass that has been cleaned, resized, trimmed and polished.   Check out the ordering tab to find their information.  

Question – What do I need to buy to get started with the 375 RAPTOR?

Answer – Order a barrel, a set of reloading dies and a power case trimmer (if you don’t already own one in your reloading tool collection).   Also be sure to have a chronograph for your load development. 

Question – Who can make 375 RAPTOR barrels?

Answer – Any barrel maker or gunsmith is welcome to offer the 375 RAPTOR, they can contact JGS Precision Tools to order a reamer and gauges.

Question – We are a company that makes LR308 uppers,  can we make 375 RAPTOR uppers to sell to our customer base?

Answer – Yes, you can.  X-Caliber Barrels has listed the 375 RAPTOR as one of their barrel offerings so you can get with them and I am sure they will accommodate your order for barrels. 

Question – In there any licensing fee or permission required for anyone to make the 375 RAPTOR?

Answer – No licensing fee or permission is required, we have already notified JGS Precision Tools, CH4D and Whidden Gun Works they are free to sell reamers and dies to anyone they want.  If you are going to use my designs, respect the name and don’t try to take credit for my work. 

QUESTION – Where can I find 375 RAPTOR loading data?

Answer – On this web site under the loading tab above

Question – Who is the 375 RAPTOR creator?

Answer – Arne Brennan, who is known for the 6.5 GRENDEL, 45 RAPTOR and 7 RAPTOR. 

Question – Who else contributed to the 375 RAPTOR? 

Answer – Scott Medesha of Medesha Firearms, who has been building AR creations for over 25 years, has been a long term contributor and sounding board for my designs after working with me to build the first 6.5mm AR-15 in the late 1990’s.

Question – Where does the RAPTOR name come from that you use for your cartridges?

Answer – The name RAPTOR has its origin as a bird of prey in hunting.  In addition, the developer’s first name has its origin as the old Norse name for “Eagle” which is also the inspiration for our logo.


Question – Can the 375 RAPTOR being used in the AR-15?

Answer – No, the 375 RAPTOR cannot be used in the AR-15 due to cartridge overall length and the maximum operating pressure of 62,000 PSI.   The AR-15 is a great lightweight carbine and should be appreciated for what it is, but the platform has limitations.   Through all the attempts to make the AR-15 more powerful with special bolts and other parts, the simple truth is the AR-10, LR-308, SR-25 and G2 is a far more robust platform which allows performance to be taken to levels beyond what is possible with the AR-15 and do so with a higher level of safety and reliability.  

Even speaking of my original design that led to the 6.5 GRENDEL in the AR-15, the 6.5×47 LAPUA, 6.5 CREEDMOOR and 260 REMINGTON in the AR-10, LR-308, SR-25 and G2 surpass anything that can be done with the 6.5 GRENDEL.  Don’t get me wrong, the 6.5 GRENDEL is what I set out for it to originally be,  a good cartridge for 200-600 yard target shooting and a 300-yard whitetail deer or feral hog gun.   I am proud of it, because it took the AR-15 and made the rifle a valid sporting rifle, but I am also very aware of what it is not.    

Question – How does the 375 RAPTOR compare to AR-15 Big Bores (50 Beowulf, 458 SOCOM and 450 Bushmaster)? 

Answer – AR-15 based big bores are all challenged by the limits of the AR-15 platform, but they are also challenged by the ballistic profiles of the bullets and the case capacities of the cartridge cases.      

Lets compare:

50 Beowulf – .499 caliber 300 grain Hornady XTP MAG bullet at 1,900 fps results in a +/- 3 inch flight trajectory of just under 165 yards with approximately 820 foot pounds of energy at that range.  

458 SOCOM – .458 caliber 325 grain Hornady FTX at 1,860 fps, results in a  +/- 3 flight trajectory just under 180 yards with just over 1,299 foot pounds of energy at that range. 

450 Bushmaster – .452 caliber 250 grain Hornady FTX at 2,150 fps results in  a +/- 3 inch flight trajectory of 200 yards with 1,176 ft pounds of energy at that range.  

45 RAPTOR – .452 caliber 250 grain Hornady FTX at 2,450 fps results in  a +/- 3 inch flight trajectory of 225 yards with energy of 1,429 ft pounds at that range.

375 RAPTOR – .375 caliber 260 grain Nosler Accubond at 2,300 fps results in a +/- 3 inch trajectory of 240 yards with 2,069 foot pounds of energy at that range.

Conclusion:  The 375 RAPTOR has 145% greater range with a +/-3 inch flight trajectory and 252% greater terminal energy than the 50 BEOWULF at that increased range.  Comparing to the 458 SOCOM, the 375 RAPTOR has 133% greater range and 159% greater terminal energy at that increased range.  On the 450 Bushmaster, 120% greater range and 176% greater terminal energy at that increased range.   

Source: All calculations performed using jbmballistics.com using advertised velocities and ballistic coefficients for each cartridge and bullet.

Question – How does the 375 RAPTOR compare to new cartridges like the 458 HAMR which is just being released in 2018.

Answer – Based on initial information released, all we can see about the 458 HAMR is that it generates 3,000 ft-lbs of energy with what appears to be a 300 grain Barnes bullet.    Based on Barnes website, the .458 Caliber 300 grain Barnes TSX BT has a .236 BC.   At 2150 fps, it will generate 3,050 ft-lbs of muzzle energy.    If we an equal weight bullet, the .375 Caliber 300 grain Game King has .470 BC, which if we held to the same muzzle velocity would have  3,079 ft-lbs of muzzle energy.   Now if we look down range, the .458 Barnes will have 2,182 ft-lbs of energy at 100 yards and 1,540 ft-lbs of energy at 200 yards.  In contrast, the .375 Sierra will have 2,603 ft-lbs of energy 100 yards and 2,188 ft-lbs at 200 yards.   Doing some quick math, the .375 Raptor firing the 300-grain Sierra Game King will have 20% more energy at 100 yards and 42% more energy at 200 yards than the Barnes .458 TSX BT when both bullets are fired at 2,150 fps.  

In summary, while it is nice to talk about muzzle energy,  what matters in the real world is on target energy and .375 caliber has vastly improved ballistic coefficients than .458 caliber and that results in superior energy on target.  Of course, the 375 RAPTOR is also a direct barrel swap on to an LR308 pattern rifle, you can quickly and easily modify a PMAG and most importantly make your own brass from .308 Winchester / 7.62 NATO. 

Question – How does the 375 RAPTOR compare to AR15’s .375 caliber offerings like the 375 WHISPER or 375 SOCOM? 

Answer – The 375 WHISPER is based on necking up a 7 BR case and the 375 SOCOM based on necking down the 458 SOCOM whereas the  375 RAPTOR is based on modifying 308 Winchester or 7.62 NATO brass, one of the most abundant cases produced worldwide,  using a single resizing step and normal case trimming.  

As a result,  the 375 RAPTOR has larger case capacity to achieve higher supersonic velocities with mainstream manufactured 250 to 300 grain hunting bullets.  In addition, the 375 RAPTOR can perform subsonic with very heavy bullets like the 350-grain Sierra Match King – a bullet that the 375 SOCOM cannot feed from an AR-15 magazine.   While the 375 SOCOM and 375 WHISPER can achieve supersonic performance, they do so with 200 grain or lighter bullets with lower ballistic coefficient bullets than the 375 RAPTOR which achieves 2300-2500 fps using bullets 50-100 grains heavier with ballistic coefficients in the .370 to .470 range extending the hunting range and increasing on target energy.

As far as SSK Industries (375 Whisper) or Tromix (375 SOCOM), both companies are more than welcome to offer the 375 RAPTOR.   No licensing fees required, just use the proper name and credit is all that I ask.

Question – Do the developers of the 375 RAPTOR have anything to do with the 375 Reaper? 

Answer – NO! We never have even spoken to the people involved in that cartridge.  We did find reading the performance claims quite interesting given we knew what performance we have been getting with the 375 RAPTOR, a larger cartridge with 50% more case capacity,  during the past 2 years of development and knew that the performance claims of that much smaller cartridge were highly suspicious. 

As historical background, Arne Brennan and Scott Medesha have over 50 years of combined experience developing cartridges for the AR15 and other platforms.  

Arne Brennan started his 6.5mm project on the AR-15 in 1997 and that projectile directly led to the 6.5 GRENDEL being released by Alexander Arms in 2004.   In addition to the 375 RAPTOR, 45 RAPTOR and 7 RAPTOR, Arne developed the 300 SKADI which propels a 230-grain Berger at 3125 fps for use in 1,000 yard F-Class.  Arne is NRA Classified High Master in F-Class Shooting. 

Scott Medesha was on the leading edge of building competition AR-15’s as early as the late 1980’s and was putting 6 PPC’s in AR-15’s in 1990 looking for ways to improve the competition performance of the AR-15.   Scott is a past member of the US PALMA TEAM and is an NRA Classified High Master in Long Range Prone Shooting. 

Question – Will the 375 RAPTOR work in a FAL or “insert another firearm name”? 

Answer – The 375 RAPTOR was developed to work in a Direct Impingement LR308 / SR-25 platform as well as Remington 700 / Model 7 Bolt Actions and the Browning BLR Takedown Lever Action.  We have not done any development using other semi-automatic rifles or using gas piston systems, nor do we have plans to. 




This website will continually be updated with additional information, data, photos and videos.