Best Caliber for SBR?

[HoosierQ]

We all know the issues with 5.56 in the M4...low bullet weight and all. What is the best (or some good) calibers for an SBR AR type of platform. 6.8spc? 6.5 Grendel? .300 Whisper? Other?

 

[gvnwst]

I would go with the .300 whisper, as subsonics work BEST with short barrels (around 8"-10" is ideal), vs the other cartridges you listed, which all do better with longer barrels.

 

[rbernie]

7.62x39 by all reports works well, if you want to stick to commerical ammo.

 

[HoosierQ]

I saw that crazy show Future Weapons where Knight designed a 6mm round for use in a SBR PDW. They noted that 5.56 was not right for their 10 inch barrel. I have, however, not been able to learn any more about that.

I am just spitballing SBRs but I don't want to shoot a long barrel caliber out of a short barrel.

 

[Dr. Tad Hussein Winslow]

.22 lr
.300 whisper
.338 spectre
.500 phantom
.50 Beowulf
.45 acp
9x19mm

some of the more interesting ones to me for SBR / PDW.

6.8 spc / 6.5 grendel / 7.62x39 are better than 5.56x45, but they wouldn't be my choice for an SBR.

 

[TehK1w1]

10MM, 7.62x25 TOK

 

[Kino74]

I have SBRs in 7.62X39 and having one made now in 5.56. I wouldn't worry about the "stopping power" of a 5.56 at the range a SBR is supposed to be effective, ie very close range. You will have a bit more recoil with the larger 7.62X69 which will affect the speed of your follow up shot so thats something to weigh in on. I like both of those two and recommend either one.

 

[The Wiry Irishman]

You will have a bit more recoil with the larger 7.62X69

I bet!

 

[HoosierQ]

I am technically ignorant on this subject (reloading and ballistics etc) but from what I have gathered here and other places it would seem that something like a 6.5x40 kind of a thing would be what would work well in an SBR coupled with the right powder. Frankly an updated version of the very kind of thing that they hoped to accomplish with the .30 carbine...only updated with more modern concepts. Obviously others (practically everybody probably) knows this stuff better than I do. But I have heard so much negative about how the M4 performs with a round designed for a much longer barrel I have to wonder. So that is where I came up with 6.5x40. Big enough bullet to deal with the lowered velocity and the right sized case to minimize muzzle flash and report, which again, I understand are prominent with the M4 shooting 5.56x45.

Or how about this...do like the .357 sig only instead of necking down the .40 case, neck down the 10mm case! That would be something.

 

[briansmithwins]

I did 9mmP for my 1st SBR which is still being processed. I'm going to cut my semi auto Uzi down once the paper comes back. Being blowback operated there aren't any problems with gas ports vs barrel length.

If you're SBRing an AR make sure you use a barrel/gas port combo that works. You just can't just use arbitrary gas port locations and get one running good. If I had to SBR a AR I'd go with 5.56 on a 14.5" barrel as that combo is known to work pretty good. BSW

 

[Dr. Tad Hussein Winslow]

What's that crazy Russian round, 9x33 or something?

 

[gvnwst]

You mean 9x39, the totally awesome totally insane super cool subsonic AP round that is in the amazing gun known as the VSS Vintorez? I hope so.