Home Forums Hunting & Shooting AR and Tactical Rifles Short barrel velocity loss

I bought a Diamondback AR pistol in 300 Blackout. 10.5 inch barrel. I took it out last week and shot sub-sonic 220 grain. I had two failure to feed issues, but I expected some teething issues. Today I ran some Hornady practice ammo, 125 grain BTHP. Advertised velocity 2175 fps from a 16" test barrel. I was really curious to see what it was from a 10.5" barrel. A bit surprised when I came up with 2111 average. About 11.6 fps per inch. Not bad. No issues.
I bought 1K once fired brass, various head stamps. I loaded up a ladder of 150 grain Hornady HPBTs over Accurate 1680. First string looked good, 19.4 grains 1803 ave 6.5 std dev. Up .2 grains, second shot of the string, I get lined up squeeze off the shot and CLICK no BOOM. Failure to go into battery. That sucker was stuck. I thought crap, I should have crimped, I set one back. So that was it, it wasn't coming out. I extracted the round when I got home figuring I was going to see a set back bullet but no. The neck on that piece of brass was really thick. I didn't resize this brass, just chamfered and loaded. I don't think sizing would have made any difference. I checked length but not neck thickness. I had one that was a btch to seat though, and when I checked it I saw the neck was all shiny from being so tight in the seater die. Should have checked them all. I'm going to have to go through all this brass. Some bargain. I also need to get a go-no-go gauge. From what Ive read, this once fired brass began life as 5.56. Don't know if that's true but it makes sense because length varied considerably.
I kind of rambled here, but I thought I'd put this info out there for anyone who's interested.

I would be real easy to tell if it was 223/5.56.
If the headstamps are NATO marked or 223, it is.
If it's mixed comercial/military, the military is likely thicker.

The one that was hard to seat is marked IVI. 10 and a circle with a cross in the center. The one that jammed was the same with 05 instead of 10. is that date of mfg?

What you have done & how you are using that brass is a recipe for disaster on several fronts.........................

MM

I agree with MM. I always separate by head stamp.

And once fired trimmed and crimp removed is different than once fired and tumbled only.

Once fired and processed (trimmed, cleaned, crimp removed) military brass is so cheap (around $100/1,000) why would you do anything but that?

Scout, as has been said. Start over with a batch of similar brass. There are a lot of variables between mfgs.
You will Save yourself a lot of wasted time and components and frustration chasing results that will never be consistent

All good advice. The packaging by Top Brass claimed load ready. The two I had trouble with were Canadian made: IVI Industries Valcartier Inc., Valcartier, Quebec, Canada NATO.

At bare minimum, throw away all of the IVI brass (unless you really feel like turning case necks).

If you're seeing a bunch of variation in case length, that's not because it came from 5.56 brass, it's because it wasn't trimmed consistently. That's disappointing, but maybe in this case it's an example of "if you want it done right you've gotta do it yourself".


150gr anything is a long way less than ideal in a 300 Blk AR, especially with 1680. If you want supersonic loads, you're better off sticking to ~130gr or less. If you want subsonic, stick to 200gr or higher. Pay attention to what the rifle's gas system is doing; trying to push the medium or heavy bullets to full power supersonic is a solid recipe for an overgassed system.

Yondering, thanks for the info. Good stuff. Yes, I thought the same thing about the brass. I hope half of it isn't IVI. I was using the 150 grain because I had it sitting around and thought I'd see how it worked out. My 308s don't particuarly like it. Good to know about overgassing the gun. I had seen a youtube video about that very thing in the same model rifle as mine. I've got some lighter bullets I can try. I'll also invest in some new brass in this caliber. Thanks again.

Glad to help. FWIW I form most of my 300 Blk brass from LC headstamp and never had issues with it, but some other brands of 5.56 are known to have thicker case walls, which translates to thicker case necks when it's made into 300 Blk. There are some lists of those brands floating around online, you might find them on the 300blktalk forum.

Forming the cases yourself is fairly easy, but it's definitely time consuming so I certainly understand the desire to buy pre-processed brass. Hopefully Top Brass will make it right, especially if you find a bunch of IVI headstamp. They may not be aware of the issue, depending how long they've been forming 300 Blk brass.

To answer your earlier question on headstamps - the circle with a cross in the center is a Nato marking that you'll see on a lot of 5.56 milspec brass. The two digit number is the year of production. Most brass with those markings has crimped primer pockets, so make sure you address that if it hasn't been done already; the crimp can be removed by either swaging or reaming.

One final note - I suggest picking up some Lil'Gun powder for supersonic loads. It's nice to work with and gives top velocity with less flash and blast than either H110 or 1680. Charge weights are usually a little smaller too so a pound of it loads more rounds.

Thanks Yondering, I went through oh, about 600 of the thousand last night, 400 were LC, 80 were IVI, some headstamps couldn't be read. I'll pick up some LiL'Gun tomorrow. I ordered a L.E. Wilson Go-no-go gauge. I'll also send a note to Top Brass. The crimps are removed.
Which brings up the question of roll crimping bullets that have a cannelure. I don't see the need really, this rifle appears to feed smoothly. What ate your thoughts?

I don't crimp at all for the 300 Blk, or anything else I fire in an AR for that matter. It isn't needed. With some jacketed bullets that have a cannelure, it probably won't hurt anything if you want to crimp, other than case mouths cracking sooner, but it won't gain you much of anything either as long as neck tension is good. With cast bullets, applying a heavy crimp can cause leading and accuracy issues.

I do use a Lee FCD to remove the flare in the case mouth when loading cast bullets, but don't do any more than just straighten out the neck.

Montana Man, you were absolutely correct. Luckily the IVI brass wouldn't chamber or things could have gone sideways in a hurry. I emailed Diamondback inquiring about their chamber dimensions- their reply - check SAAMI specs. Well ok I hope the chamber was cut to spec but I was hoping for a little more detail, no matter, I bought a Wilson min SAAMI chamber gauge. After sizing I run the brass through the gauge then again after seating. Works great. Moving the rest of this discussion to reloading. Have information about shooting results using Nosler 125 grain.