12.5" Pistol/SBR caliber choices

[LRRPF52]

If you have lighter weight hanging plates, it will move them a little at 400yds. I like watching my sight picture and seeing very solid impacts, then hearing a loud “WHACK”, vs a wimpy little “tink”. Gives me more of a dopamine hit.

I’ve shot so much 5.56 since the 1980s and put it through DM courses for decades, so it doesn’t really do much for me personally, plus I have tinnitus pretty bad. Wind drift sucks on it as well, and I deal with wind here in the Mountain West Region a lot. If you could load 75gr ELD-M and mag-feed it, it would be a bit better for hit probability. The new Nosler, Hornady, Sierra, and Berger offerings for mag-feed allowable COL bullets with higher BCs inches towards that a little more, but nothing like a 6mm or 6.5mm.

Even the 6mms don’t do it for me once you start getting past 600yds in full value winds. Not very loud, not very impactful on steel. Same with 107gr SMK in 6.5mm. Easy to hit with and very flat-shooting, but not enough energy on-target, or as audible for me.

I had my 7yr-old son smacking the steel with 12” and 17.6” Grendels at 400yds with ease his first time shooting that distance last Thursday. 400yds is boring with pretty much any barrel length in Grendel.

Circle plate near the center is 400yds, buffalo is 780yds in this image through my Leupold 10x binos mounted to the tripod:

 

[Hoody2shoes]

That brings up something we aren't really discussing but should be-the ever popular use case. My 12.5 is my fighting rifle if you will. If I was hunting or trying to ring steel further out I'd have picked a different caliber.

 

[LRRPF52]

That brings up something we aren't really discussing but should be-the ever popular use case. My 12.5 is my fighting rifle if you will. If I was hunting or trying to ring steel further out I'd have picked a different caliber.

12.5” 5.56 is ideal for that. Any more barrel is kind of a waste of space since you aren’t launching any projectiles of substantial weight, and even 11.5” and 10.5” will get it done within common distances. I love 11.5”-12.5” 5.56 NATO. I doubt I’ll ever pay for anything longer again in that caliber. Makes zero sense for me, and the 20” arguments fall apart really fast when you run the numbers and look at how anemic they all are as you get past 200-300yds.

The only time you’ll catch me doing a new 20” is if I do a Retro Colt model 656, and even then, I would be tempted to do a shorter barrel with AEM5 and do it in 6.5 Grendel instead. I would conceal DCM free-float it too. This was the first 5.56 flat top upper receiver design made by Colt, with an integral BUIS built into the receiver, hooded front sight.

It's one of the most overlooked Retros, not known to many, but ahead of its time as you can see. Would have made a great DMR in the inventory 30-40 years ahead of the Mk.12.

 

[Hoody2shoes]

I think the only two 556 lengths worth messing with are 12.5 and 18. If you need longer you need a bigger bullet, if you need shorter you need something better at shorter work.

Like I said 2650 with a 12.5 and mk262's is plenty of 556 for me. That gets me north of 700y before playing with the transonic bullshit.

 

[LRRPF52]

I think the only two 556 lengths worth messing with are 12.5 and 18. If you need longer you need a bigger bullet, if you need shorter you need something better at shorter work.

Like I said 2650 with a 12.5 and mk262's is plenty of 556 for me. That gets me north of 700y before playing with the transonic bullshit.

2650fps with 77gr from a 12.5” 5.56 sounds really fast.

Looking up 20” Service Rifle load data with 75-77gr, half of the loads won’t even get there in Hornady’s manual.

Sierra’s manual shows 5 out of 11 powders that will get there in a 20” AR-15.

What trade-offs did Core make with their bore, chamber, and rifling to achieve those speeds?

You could tight-chamber and use shallow rifling with a specific land/groove ratio, but how long will it last? That’s a spicy 12.5” either way.

What does your brass look like?

If that system is sound, it has my interest for 5.56mm.

 

[Hoody2shoes]

2650fps with 77gr from a 12.5” 5.56 sounds really fast.

Looking up 20” Service Rifle load data with 75-77gr, half of the loads won’t even get there in Hornady’s manual.

Sierra’s manual shows 5 out of 11 powders that will get there in a 20” AR-15.

What trade-offs did Core make with their bore, chamber, and rifling to achieve those speeds?

You could tight-chamber and use shallow rifling with a specific land/groove ratio, but how long will it last? That’s a spicy 12.5” either way.

What does your brass look like?

If that system is sound, it has my interest for 5.56mm.

The system is sound, brass is fine. I've paired several criterion barrels (both core and hybrid) and a couple other manufacturers with the Superlative Arms piston kits. This combination has created faster than average results EVERY SINGLE TIME. I have a thread on here I've been updating as I try more barrels/combinations. I'm going to do a di/piston test back to back on a 12.5 core eventually as I'm about 95% certain the piston kit is creating higher pressures and faster velocities

That particular barrel I'll be testing has around 7k through it, a mixture of steel, low power remains, and spicy full power stuff and has held about .85moa with mk262 at 100y for 5rnds. I was going to do the test last month but I broke a hex key off in the set screw and am still figuring out how to remove it lol

 

[LRRPF52]

One problem I run into now is every time I look at adding another 5.56 to my armory, my only interest is in 10.5” to 12.5” barrel lengths.

I then look at the fact that I get 2700fps with the 90gr Federal TNT factory ammo from my 12” Grendel and lose interest pretty quick. I’m not burning it down though in high volume CQM sessions like I’m used to with 5.56 either.

 

[Hoody2shoes]

I get it. Just posting data not trying to convince you of anything