CCI small rifle primers get blown out of my 300 BLK subsonic rounds

My gun is a 10.5" 300 BLK AR Pistol with a Deadman S Suppressor. Daniel Defense barrel, BCM bolt.

I’ve reloaded some Hornady bullets in some recycled 300BLK and .223 Brass. They shoot properly, but almost every other round ends up blowing the primer out. This didn’t happen with store bought 300 BLK, so I guess my brass is too small for the primers. So far I’ve been told to:

  • Trash the cases, save the powder and bullets
  • Use Wolf primers
  • Use some kind of Primer sealant (I thought this was for moisture control, not adhesion)

What can I do to prevent wasting these rounds with the least amount of effort?

Check your load. It is probably too hot. Back off in 1/2 gain increments until the problem is solved- don’t go below starting load in the load data.
Look for indications of high pressure. For example flattened primers.
Double check OAL. But remember you are seating the ogive.
You might be seating the bullets too far into the case. Or, too far out and the bullet is “stuck” in the lands. It is suggested to uniform the primer pocket before seating the primer.


I am currently AT the starting load in the load data, since I’m using the heaviest subsonic bullets. I’ll check the lengths as I fire to see if there is a pattern there.

Take off the can as you figure things out.

What is your load? Bullet, powder, OAL?

If you have a pile already made up that you don’t want to waste, you might try changing gun elements of the equation — in a different pistol? in a rifle-length barrel? without the suppressor?

That’s just a wild shot, so to speak. If you aren’t seeing conventional overpressure signs but just primers dislodged from pockets by firing, my suspicion leans to oversize primer pockets. I don’t know whether any brand stakes primers these days or uses primers with a different spec. Have you sorted the brass to see if the problem occurs across all brands, or in re-formed vs original caliber brass?

I don’t know how to shrink an oversize pocket, so if you can’t find a primer sized to what you have the cases may be waste. Just my thought — good luck.


If you’re at the bottom of the load, maybe check OAL or for oversized primer pockets - both previously mentioned.

If you are using some sort of primer pocket swaging tool, check that it’s adjusted properly. It should just take any crimps out and slightly bevel the edges.

You might consider downloading the Hornady Reloading manual to your phone. It is only $20.

Are you starting with modified 223/556 cases or 300 ACC Bk cases? In any event double check shoulder set back and proper head spacing. In other words, make sure you are using cases that meet specifications.

What is the source of your reloading data? This is important.

How many times have these cases been fired? When you were seating the primers were they going in REAL easy? If so, that is a clue that maybe the case must be put in the recycle pile.

It would be interesting know your experience at reloading and the equipment/tools you are using.

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Blown primers are indications of TWO things

  1. Over pressure while the bolt is locked
  2. Loose pockets

Suppressors are notorious for causing strange gas reactions. There are some that advocate for opening the gas port on the barrel if you are using a can. I would first ensure that your gas block is as aligned as humanly possible, factory doesn’t mean all is well cool and scooby with the gang. If you are running reloads you need to check them can on and can off as a can is essentially giving you and additional couple inches of barrel as far as velocity goes but that doesn’t move your gas port. @Mike270 has some very good points that you would do well to consider.

You may wish to try a “pig tail” gas tube to slow down the gas or an adjustable gas block so that you can tune the stick for “Can” - “No Can” operations.

Please take a look at your bolt face. If there is a “ring” the size of a primer etched into your bolt face you are essentially plasma cutting your bolt face through over pressure rounds. You should also look for “swiping” on the base of the casing (it looks like circumferential rings around the base) which indicate that the cartridge is being extracted while under pressure and is being scored by the bolt face.

Loose primer pockets after firing are a harbinger of doom in a gas gun (or a bolt gun) you will start to find stress cracks in the necks (they usually show up first due to case hardening or over sizing) and case head separations or cracks at the belt lines. Catastrophic failures have been know to happen.




??? What happened to guns that just go bang every time you pull the trigger??? :boom: :boom: :boom:

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Everybody tried to make a better 1911. :heart_eyes: :sunglasses: :crazy_face: :rofl: :rofl: :rofl:



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I wasn’t paying close attention when working up loads for 300 blackout a few months back. I was using Hodgdon CFE BLK powder and working up, trying to get a 7.5” pistol to reliably cycle. Sierra put Shooters World Blackout directly above CFE BLK and somewhere along the way while working up, I started setting my increments for the wrong manufacturers powder. I never encounter Shooters World powder and seldom pay attention to their products. Blackout and CFE BLK both make me think Blackout in my thought process, especially with the cartridge being called Blackout as well. I didn’t blow any primers by going over max in this instance. But I was not paying full attention and made a mistake. (I never got reliable cycling.) My point is that it never hurts to double and triple check everything.

I usually grab my Lee manual first and I appreciate the powder manufacturer abbreviation ahead of the powder type. It’s an absolute necessity with the 4895’s since multiple manufacturers use the same number. Could Sierra have added the extra information, sure, but it’s still my mistake at the end of the day. They could have at least not put the two powders with similar names so close together in the listing.

300 Blackout is a relatively small capacity case, and with long heavy bullets the case capacity is even less. Seating shorter than the minimum overall length is gonna raise pressures much quicker than larger bottleneck cases.

We would still like to know the recipe you were using.

Have you disassembled any of your loaded rounds and checked the weight of the powder charge?

Does the powder in the cartridges look like the rest of the powder In the original container?

I suspect the brass is the culprit. I don’t think you would have been firing multiple loads hot enough to expand primer pockets without noticing something seemed off.

Were the loads staying subsonic?

What’s the history of the brass? Did a previous owner damage the brass removing the military primer crimp?


The brass is a huge hodgepodge of random brass. Some 300, some I trim from 223… I’m gonna go to the range and do a test to see which lengths cause the problem since I think the seating depth is the culprit here. I did find a “ring” on the bolt face that I don’t see on my other bolts, so I think that’s the issue. The powder is exactly what it should be according to the bullet manufacturer specs.

This is the point I needed. I think I’ve got short lengths, and too much seating depth. Gonna confirm at the range.

What bullet, powder and charge weight are you loading?

One other thought since we still don’t know exactly what you are loading other than subsonic with mixed brass and CCI primers.

Winchester 296 / Hodgdon H110 can get erratic with reduced loads. For starting loads, some manuals only recommend a 3% reduction from maximum with that powder vs a 10% reduction with other powders.

Haven’t heard anything from you in a couple of days. Hopefully you you haven’t damaged your firearm or injured yourself!