300BlkTalk

Hello there. I am having a lot of trouble cycling lightweight rounds. I think I have a bolt bounce situation I’m not sure? I have a 9 inch BCM barrel opened the gas hole to .10" and I’m running a JP H2 spring. I have an auto bolt carrier group BCM. I can chamber a round from the magazine by using the bolt release but the next shots I will have to use the forward assist, and all the shots following. Commonly a round will get stuck behind the chamber at about a 45° angle. It will almost always do this on the last shot. I make my own bullets so my choices of bullet weight is unlimited. I was hoping for about 115 grains for the supersonic, and about 225 grains for the suppressor. These bullets use the same profile as a Sierra match King, 6.5s ogive. I’m still waiting on the tax stamp yet so all I’m worried about right now or the supersonics. Any advice or help you would have at getting this little rifle to run would be greatly appreciated.

ETA I have been using H-110, LilGun, and CFE BLK. Not happy with the CFE, as Im only getting 1600ish fps with everything from 100 to 115 grain rounds. I'm seeing low 2,000fps with the other two.

Knowing complete load data, if the bullets are cast, jacketed or solids, and if it happens with all loads or just one of the powders, as well as bullet diameter if cast, would all be good things to know to be able to help.

dellet wrote: ↑Sat Mar 30, 2019 12:13 am Knowing complete load data, if the bullets are cast, jacketed or solids, and if it happens with all loads or just one of the powders, as well as bullet diameter if cast, would all be good things to know to be able to help.

^^ this.

we need as much specific data as you can give to diagnose the issue. shoot for "i hope its not too much" and we'll probably hhave the right answer for you.

you dont mention what buffer you have in the gun. knowing that should be helpful




have you tested it with factory ammo yet? if you havent - i suggest getting a box of the Remington 120gr OTM's from walmart. if your gun wont cycle those properly - you pretty much know at that point your issue is the firearm and not ammo.

The bullets are copper jacket, lead core. 6.5s ogive, and any weight needed can be built, and I have a choice of concave, flat, or boat tail designs. I can use brass jackets too, but Im not really set up for those in all weights yet. I roll my own.

I have CFE BLK, LilGun, H-110, H-4198, and IMR 4227 to choose from.

It's using a JP captive spring H2. However, this morning I got it to cycle both a 110 grain and 115 grain with LilGun and CFE, after replacing one of the tungsten weights with a steel one, (there are two available) , but when I shot a 168 grain subsonic (12.1 CFE BLK), it wouldnt chamber the next round. I was shooting without a can.

It shoots 168 subs just fine with the H2 setup.

I've seen that pop-up problem when too short, or too long a round gets "wacked" really hard in a mag with a weak spring and bounces off the front of the mag. Usually short rounds, because short rounds use less powder and that's where I seem to play most. This is different than "bolt-over-base", which usually means slow things down, though they look similar. Unless your already too slow. With the pop-up the bolt catches the base. Both can happen at different points in the same mag. So it's probably a bunch of things, all contributing. A few things to try.

Knock the burrs off your magazine lips.
Get a HD mag spring.
Use a 300 Blk mag, modify a PMag or try a GI Mag. GI mag are smoother inside.
Search for proven "Over-All-Lengths". One of the first things I look at online.
A heavy recoil spring and light buffer can speed things up, so go "buffer before spring" to slow things down, but they work together.
To much gas can be tough to tame with just a spring and buffer. If your brass is landing way out in front of you, you need to "control" your gas. It gets worse suppressed.

I'm guessing the mag spring is too weak to overcome the friction from the ogive rubbing on the 556 "shoulder guide ribs" and the big fat bullets are bouncing off the constriction at the front of a 556 mag.

IMR 4198 never worked well for me (except in heavy 458 SOCOM and light 556... what the heck?). Too bulky in 300 Blk. Others like it though. Lil-Gun never shot best for me. It burns hot. Others like that too. IMR 4227 is my go to for heavy Subs right now, though I've set my sights on N110. Pretty mellow powders for a new or tight gun. H-110 likes it hot. Can squib on light loads and has for me. This is still my "go-to" for fast light bullets. A1680 makes all kinds of gas, it's good for trouble shooting, but loud and dirty suppressed.

300 Blk operates across such a wide range, it seems there is always something that doesn't work. Have fun.

I second the things said above.

When using odd bullets, try making some dummy rounds that are a couple tenths shorter OAL, or longer if that isn't riskng a jam into the leads issue.

See how it feeds off of the bolt release or sling shotting, or deliberately sling shotting from a point short of the bolt release. Then try it agian but take out the mag and whack it so that the cartridges are at the front of the mag rather than the back.

Some times the different nose profiles bounce off of the feedramps and glance at strange angles. Tweaking how much of the case is supported by the feed lips at the point it hits the ramps can make it work. two of the bullets I have played with have a long mode they feed in, and a short mode, but have a zone in the middle where they intermittently do the pop up thing. The pop ups will leave jacket deposits on the 10 and 2 o clock positions of the chamber from how they glanced off of the feed ramps. Sometimes if a cast bullet is tricky to diagnose, I will paint it up with a paint pen and then chamber it wet. Paint will show anywhere it is hitting, and will also show very clearly if the bullet is getting gouged or beat up anywhere.