Cartridges and Calibers for SBRs: The Complete Guide

With the elimination of the $200 tax on SBRs and suppressors in the Big Beautiful Bill, it has become a little more financially attainable to purchase either or both. There are also lawsuits pending to eliminate the onerous paperwork to buy one of these. With a larger segment of the shooting public beginning to jump into the SBR and suppressor market, a discussion of ammunition considerations for these platforms is timely. SBRs will run on generally any ammunition that is made for their chambering but most of the rifle cartridge ammunition is not optimum for several reasons I will explain below. There are definitely criteria that apply and can be used to handload or evaluate factory loaded ammunition for SBRs. I will say up front, outside of pistol-chambered SBRs there is little commercially available ammunition that is really truly tuned for efficient operation in an SBR. Generally speaking, the criteria that goes into well designed SBR ammunition also carries over into the performance, ease of use and longevity of a suppressor on an SBR. With all that said, I’d like to briefly discuss what is going on in the bore of the gun when a round of ammunition is fired and what effects that has on a gas operated SBR and a suppressor. For the discussions below I am going to extend my definition of an SBR to include fixed breech rifles up to 16” barrel length. This opens up more choices of firearms and more cartridges.

In Bore Ammunition Performance

We’re all pretty familiar with what happens to a bullet in bore. It has high pressure applied to the rear of it, it is engraved by the rifling and is spun up and exits the bore. For a subsonic round the twist rate has to be substantially faster than that of a normal barrel length higher velocity firearm. At subsonic velocities the bullet exits the barrel at near the peak of the drag on its drag curve and correspondingly high overturning forces on the bullet. The slower muzzle velocity results in a slower spin on the bullet and a less stable bullet. These two things, coupled with usually long heavy bullets requires very fast spin rates to stabilize subsonic rounds.

The following interior ballistics discussion this is aimed at primarily suppressed, AR platform SBRs. I am going to assume that most people going to the trouble of legally buying an SBR will also acquire a suppressor for it. Because of the gas impingement operation system of the AR, it has some special ammunition requirements for efficient and trouble-free operation. These considerations don’t necessarily apply to a fixed breech system but many of the SBR ammunition design criteria benefit the performance of a fixed breech SBR platform, particularly with a suppressor.

When a round of ammunition is fired, the propellant begins to burn and rapidly increases pressure until the projectile begins to move faster than the propellant can produce gas. In a pistol cartridge the peak pressure is usually within approximately the first 1” of travel and then the pressure begins to fall. In a 223/556 cartridge the peak pressure is usually generated after the projectile has travelled about 2.5” depending on the bullet weight and propellant speed. In a magnum rifle cartridge with a heavy charge of slow propellant, peak pressure occurs at 4” - 5” of projectile travel. The propellant charge in rifles are generally designed to be completely consumed somewhere in the 20” – 24” range.

If a barrel is shortened to say 10 inches for a rifle cartridge and standard ammunition using propellant designed to burn out in a much longer barrel is fired, it creates some rather undesirable things going on in the gun. In an AR platform an SBR is usually using a rather short gas tube and the gas port is pretty close to the chamber. This makes it much more difficult for the firearm designer to not have an over-gassed system that opens the bolt rather early with a lot of pressure still in the case. This is hard on cases and limits their life. The gas system will dump a lot of high pressure gas into the action because the port is so close to the chamber. The short barrel results in very high muzzle exit pressures and a lot of unburned powder at the muzzle. With a standard .223/5.56 round there is about 30 – 40% of the propellant unburned in a 10” barrel depending on the load. If the gun is not suppressed it results in a very large and loud muzzle flash. Figure 1 shows a typical projectile base pressure versus travel plot for a rifle cartridge. Base pressure is what the actual pressure is at that point in the barrel as the propellant burns and the projectile travels down the barrel.

Notice the large difference in pressure at a 20” muzzle location for a standard AR 15 versus a 10.0” muzzle location for an SBR AR platform, 7,800 psi versus 17,500 psi. Also take note of the pressure difference between a gas port at 14” and 7”, 11,500 versus 23,000 psi. If the gun is suppressed there is much higher pressure gas going into the suppressor and a lot of unburned powder versus the 20” barrel. This results in a lot of carbon and residue in the suppressor making cleanup more frequent and more difficult and probably reduces the life of the suppressor.  This higher pressure in the suppressor results in much more gas and unburned propellant fragments being forced back into the action by the gas system. This in turn means the action needs to be cleaned more often and is more difficult to do. The factor of over 2 times difference in port pressure between a 14” location and a 7” location will drive the action much harder and open the bolt much sooner in the ballistic cycle. I’m not saying that an SBR won’t run on regular ammunition, because it will. It’s just not optimal.    

Criteria for Well Designed SBR Ammunition

There are some rules of thumb that can be applied to properly-designed SBR ammunition. In general, ammunition for an SBR should be heavy for the caliber bullets. A properly designed SBR load will use lower charge weights of a fast powder for the bullet weight in order to have more efficient burn out and lower port and muzzle exit pressure. This will usually result in this load being slow and maybe not functioning reliably in a normal length barreled firearm. However, this load will usually outperform a standard load in the SBR for velocity because it is much more efficient.  A heavier bullet moves down the barrel at a significantly slower speed than a lightweight bullet. It’s higher inertia resists movement more and requires more of the propellants energy to accelerate the projectile resulting in the propellant burning more efficiently. Figure 2. shows the generic difference between the base pressure curves for a heavy versus a light bullet with a standard speed propellant and a heavy projectile with a fast-for-projectile-weight propellant.

Notice the base pressures for a light bullet are higher than the heavy bullet. Light for the caliber loads at full velocity with “standard speed” propellants will result in higher port and muzzle exit pressures. Fast propellants can be used with light projectiles to reduce bore pressures, but many times the reduced charge weights along with a light bullet can result in unreliable functioning. It is much easier to find a happy middle ground with a heavy bullet and fast for projectile weight powder at reduced charge weights as compared to a “standard” speed propellant.

Generally speaking, if a load shows full velocity for the bullet weight from a standard length barrel, I would argue it is not optimized for an SBR. A true SBR load is strictly designed around a shorter barrel and may not have enough gas volume in a longer standard rifle length barrel to reliably run the gun or produce full velocity for that bullet weight in the standard length barrel.

The optimum loads for an SBR, especially a suppressed SBR, are heavy-for-caliber bullets with fast (for the bullet weight) propellants. This combination results in lower, more manageable port pressures, better action timing, and lower muzzle exit pressure. These things combine to provide a better functioning and cleaner running firearm and suppressor, less recoil and longer case life if you reload.

Suggestions for SBR Cartridges

22 Rimfire

Don’t look past the 22 Rimfire. Firearms are reliable and relatively inexpensive and of course available as semi-auto or fixed breech. Barrels are easy for a gunsmith to shorten to 16” and thread the muzzle. Virtually every 22 Rimfire ammunition provider offers several subsonic loads. Some manufacturers offer standard velocity loads that are advertised as suppressor friendly. Most standard velocity 40 grain projectile .22 LR loads are suppressor friendly as the propellant is completely consumed by 14” – 16” of travel. 22 Rimfire provides a lot of inexpensive shooting, and would be a very good entry into the suppressor game. Additionally, subsonic .22LR loads in a suppressed firearm are virtually silent.

9 mm Luger

The 9 mm Luger is a no brainer. It meets all the criteria discussed above with virtually every load out there, as it was developed for a 4” barrel. Most any 9 mm rounds propellant is totally burned in 4 or 5 inches of barrel travel. I you want subsonic ammunition, you will have to stick with non +P 147 grain loads. You may have trouble getting factory 147 grain loads (even those advertised as subsonic from a pistol) to be subsonic in a 16” barrel, as the extra barrel length over a 4” standard barrel will result in about a 150 fps velocity gain. You may have to handload to get subsonic performance. There are a number of 9 mm chambered carbines available with 16” threaded muzzle barrels. 

223/556

This is a popular chambering for AR platform SBR rifles. It is also tough to get really optimized performance with this cartridge in an SBR. The only true optimized load I have experience with, that is also commercially offered, is the Hornady 556 NATO SBR load. It is loaded with a 75-grain bullet that was designed solely for this load and it’s velocity range. It is loaded with a modified shotshell powder that results in a rather low charge weight but still fills the case because of the low density powder. The load gives about 2,200 fps from a 10.5” barrel. It is very clean and very easy on suppressors mounted on an SBR. This load is only marketed to law enforcement currently, but you may be able to find it out there. Hopefully Hornady will make this load available to the commercial market. 

There are several other loads out there that are marketed as appropriate for SBRs, but they are generally standard loads that are marketed as such because the bullet provides decent terminal performance at the SBR muzzle velocities. 

300 Blackout

This is probably the most popular chambering in SBR platforms and justifiably so. 300 Blackout is the standard bearer for SBR ammunition and subsonic ammunition. It was designed from the ground up for this application and is highly developed with many ammunition choices available. Loads are available that are subsonic and lighter weight supersonic loads. So much has been written about this cartridge I won’t waste any more ink on it here.

338 ARC

The 338 ARC follows along the same lines of the 300 Blackout. It was designed from the ground up as a subsonic SBR round. It would be particularly interesting for someone wanting to hunt with an SBR as the 307 grain Sub-X bullet provides expansion and tons of penetration. Lighter projectile weight supersonic loads are also available.

Don’t Overlook a Fixed Breech Firearm

The return of the 1894 Marlin by Ruger opens up some interesting options for someone wanting to dabble with subsonic loads and a suppressor, but maybe not go the whole way to acquiring a full on SBR. Ruger offers the 1895 Trapper model with a 16-inch barrel and the muzzle threaded for a suppressor. It offers models chambered in .357 Magnum, 10 mm Auto and .44 Magnum. You won’t find subsonic factory ammunition for any of these loads at this point in time, but a motivated handloader can easily load the heaviest for caliber bullets in each caliber to subsonic velocities. .38 Special or .44 Special can certainly be loaded to subsonic velocities if they will function through your gun. Virtually any load in these calibers will work well because they are designed for barrels much shorter than 16 inches. These guns also offers some interesting options to someone wanting to hunt quite in short range hunting environments. Most of the hollow point bullets available for these cartridges will expand in the 1,000 – 1,050 fps muzzle velocity range and provide deep penetration. The Hornady Subsonic line also offers some other interesting fixed breech options with a 30-30 Winchester, 350 Legend and 45-70 subsonic loads among several others. Throwing the old Thompson Center Contender and Encore firearms into the mix opens up a lot of possibilities for someone wanting to tinker.

Reloading Comments

I’d like to close with some suggestions for loading for SBRs. For fixed breech firearms this is pretty simple. Most any reloading handbook will have load data for whatever caliber you choose. If you are shooting suppressed, the shorter the barrel the heavier and faster propellant you want to use for efficient performance. As I said above, this approach will save you pain in cleaning things and probably extend the life of the suppressor.

For AR platforms, things get a bit more complicated. The same rules apply that the shorter the barrel, the heavier the bullet and faster the propellant you want to use. This has to be balanced against making sure the gun runs reliably. You will need to function test loads to make sure the gun runs reliably and go from there. A suppressor will make it much easier to get the gun to run on faster, lower charge weight propellants because the suppressor forces more gas back into the gas system. This is a much different mindset than is usually used in handloading. You aren’t trying to get maximum velocity, you’re trying to get maximum efficiency. 

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