How bad can too much powder make accuracy?

[destrux]

Dealing with a CETME, roller locked semi auto in 7.62x51. Rifle shoots just fine with commercial jacketed ammo.

This rifle needs a very brisk load to function properly. I worked up a load using the Lee C312-155-2R bullet (powder coated and then gas checked and sized to .309) and IMR 4046 starting at 35gr and found it will not cycle reliably till I reached 40.0gr. I didn't shoot for groups at all till I got to the charge that would cycle the rifle.

When this rifle doesn't fully cycle, it mauls the brass. Crushes it like a spent cigarette butt in the action, even if you single load each one. What's worse, it occasionally breaks the extractor spring when it does this. So accuracy testing lighter loads is out.

The problem is, at 40gr the accuracy is worse than horrible. I actually can't believe a gun could shoot this badly. The group is literally over 15 inches at 10 yards. Yes. That bad, no exaggeration at all.

My main question is... is 40gr way too much to expect a cast gas checked bullet to endure? Am I just flat out expecting it to take too much of a powder charge there? Base failure maybe, even with the gas checks? or is the gas escaping around the bullet as it goes through the tight part of it the conical flash hider burning the sides of the bullets or throwing them off in a random direction?

The funny thing is, none of the holes in the target are showing any keyholing. Just horrid accuracy.

I'm not getting any pressure signs on the cartridge, actually it's not even expanding the brass into the chamber flutes like it does on the commercial ammo. Recoil is also mild.

I haven't been able to recover any bullets from the berm to see if they have any clues. I did find some gas checks flattened out laying around the berm. They were flat but intact. I suspect those are from the few that hit the steel plates and the rest of the bullet vaporized.

Thoughts?

[Larry Gibson]

destrux

"My main question is... is 40gr way too much to expect a cast gas checked bullet to endure? Am I just flat out expecting it to take too much of a powder charge there? Base failure maybe, even with the gas checks? or is the gas escaping around the bullet as it goes through the tight part of it the conical flash hider burning the sides of the bullets or throwing them off in a random direction?"


No, 40 gr is not way too much to expect a cast gas checked bullet to endure. It is however, way too much for a cast bullet to endure in a 10" twist barrel if one expects accuracy also. The problem of inaccuracy is not caused by any of the factors you mention. I will tell you in a PM if you'd really care to know. Certain members here go ballistic (pun intended) when I mention the actual reason.

"The funny thing is, none of the holes in the target are showing any keyholing. Just horrid accuracy."

What you're observing there is a classic indicator of the actual reason accuracy is poor. The bullets are perfectly stable but accuracy is "horrid". I suggest you try a 311299 or perhaps the Lee 200 gr bullet over a medium powder such as 4895 with a Dacron filler. You will find very good accuracy at a lower velocity. The heavier bullet and medium burning powder will allow sufficient gas port psi for functioning while maintaining the lower velocity for accuracy. You might also post your problem/question on goodsteelforum.com for a complete explanation.

Larry Gibson

[OS OK]

destrux

"My main question is... is 40gr way too much to expect a cast gas checked bullet to endure? Am I just flat out expecting it to take too much of a powder charge there? Base failure maybe, even with the gas checks? or is the gas escaping around the bullet as it goes through the tight part of it the conical flash hider burning the sides of the bullets or throwing them off in a random direction?"


No, 40 gr is not way too much to expect a cast gas checked bullet to endure. It is however, way too much for a cast bullet to endure in a 10" twist barrel if one expects accuracy also. The problem of inaccuracy is not caused by any of the factors you mention. I will tell you in a PM if you'd really care to know. Certain members here go ballistic (pun intended) when I mention the actual reason.

"The funny thing is, none of the holes in the target are showing any keyholing. Just horrid accuracy."

What you're observing there is a classic indicator of the actual reason accuracy is poor. The bullets are perfectly stable but accuracy is "horrid". I suggest you try a 311299 or perhaps the Lee 200 gr bullet over a medium powder such as 4895 with a Dacron filler. You will find very good accuracy at a lower velocity. The heavier bullet and medium burning powder will allow sufficient gas port psi for functioning while maintaining the lower velocity for accuracy. You might also post your problem/question on goodsteelforum.com for a complete explanation.

Larry Gibson

I am quite surprised at your response to this members question. The only reason I clicked on this thread is because I wanted to hear the answer put forth by such a revered and respected man as yourself. I don't go back far enough to remember you for anything more than the scuffle that got goodsteel removed from our midsts and don't recall that event clearly as it makes no difference to me at all.

With the welcome that all the members made for you...all that...well, I'll just say I was surprised and a little bit embarrassed for you as I saw so much sucking up...I didn't think you appreciated all the attention, however that's just my response.

I am surprised that you don't just speak your mind up front loud and clear...share your honest and hard earned opinions...why should you care what others may say? Are we not here for the sharing?

[Larry Gibson]

Perhaps the RPM Threshold sticky should be put back up (it was removed when I was banned).....the sticky that just had the explanation in it and not all the "discussions"?

Larry Gibson

[Scharfschuetze]

First: Welcome back Larry! It was a nice surprise to see your avitar with today's date.

Second:

This rifle needs a very brisk load to function properly. I worked up a load using the Lee C312-155-2R bullet (powder coated and then gas checked and sized to .309) and IMR 4046 starting at 35gr and found it will not cycle reliably till I reached 40.0gr. I didn't shoot for groups at all till I got to the charge that would cycle the rifle.

Loading cast for a semi-auto rifle is often a question of compromises between function and accuracy. I have no experience with your retarded roller locking recoil operated action with cast bullets, but given the fact that its function depends on recoil and not gas pressure, your best bet is to follow Larry's advice above and go with the heavy Lyman 311299 or the Lyman 311284 bullet designs. They will provide more recoil impulse than your Lee 155 grain bullet when shot at the same velocity. My sweet spot for most military rifles including gas operated specimens has always been between 1,800 and 1,900 fps. In addition to the increased recoil impulse available from a heavier bullet, your 1 in 10 or 1 in 12 twist will also be more amenable to the longer projectile as far as accuracy goes.

I've read that there are different cam angles for the roller locking system of the CETME and G3 rifles. Perhaps a search on the web for info regarding a different bolt with milder cam angles would help. The Spanish ball ammo for the CETME design was pretty mild in comparison to NATO standards, so your rifle may already have a bolt optimized for lower powered ammo. Just a guess.

I wish you success with your efforts. I think that you've probably chosen a very difficult project so keep us informed.

[nekshot]

Os Ok, if that is the only reason you clicked on then you input is total childish and we are men, so hush up a bit and be considerate as men ought to be!

[Scharfschuetze]

Third:

In thinking back to the CETME descendant G3 rifles that I had access to in Central America, I vaguely remember the rifling to be some kind Polygonal rifling. At any rate, I don't recall it being conventional. What kind of rifling does your CETME have? If it's conventionally rifled, you should have some chance of success. I don't know about cast in the polygonal style of rifling.

Fourth:

I am quite surprised at your response to this members question.

OS OK, I was surprised at your response. I thought the thread was about a CETME rifle and cast bullets. Perhaps a PM would have been a more mature venue for such a discordant rant.

[Greg S]

I have a CETME and I feel based on the design, it's a poor candidate for a boolit launcher based on the delaid roller design, polyganol rifling and a fluted chamber. It's tuff enough to resize brass without loosing 20z30% to cracked bodies and necks.

[destrux]

This one has been built using a PTR91 barrel which uses conventional 6 groove RH twist rifling.

The roller delayed actions like this one actually use the rollers as a mechanical disadvantage system to falsely increase the weight of the bolt carrier by a large ratio for the first 3/8" of travel. That's largely how it "locks" the action. So changing the ramp angle of the locking piece or changing the bolt carrier weight are the two ways to effect the pressure that it unlocks at and how quickly it unlocks. Higher degree angle and lighter carrier weight both make it unlock easier and sooner.

The model I have does have the earlier model lighter weight "Model B" carrier from when the CETME rifle was issued with low pressure 7.62 CETME ammo before Spain went to NATO spec ammo but the 60 degree CETME low pressure locking pieces are not available in the US. The locking piece in mine is the full pressure 7.62 NATO locking piece of 50 degrees. HK locking pieces don't interchange with the CETME the critical dimensions are different. I have considered getting a spare locking piece reground to a different angle for low pressure ammo but it's not a simple operation since the locking piece effects the bolt gap (which is as critical as headspace in this type of rifle).

I trust the judgement of any of the long time members here, so I will give the heavier longer bullets a shot. Thinking about it now it makes perfect sense for several reasons. I hadn't considered that the 155gr might not be compatible with the twist rate at that velocity.

[Ford SD]

308 in a semi
you need to look at Bruce B Work
http://castboolits.gunloads.com/show...pringfield-M1A

note he used a 1/11 twist

from my testing I had a 1/10 twist and my loads went shotgun at a lower velocity than the data that Bruce shows

pick a bullet that worked good for bruce or a similar shape/ weight (the heaver weight works better over 160g)

Dacron in a load that is iffy will make it work better (reduce and work up)

[destrux]

I got this sorted out and found a load that's working really well.

I got the Lee 200gr mold and with powder coat and a gas check and a 37.6gr charge of IMR 7383, which is a full case in NATO brass. I got groups that are about as good as I think I can expect from this rifle, less than 1" at 25M. I didn't get to shoot it at the usual 100M yet because I haven't had time. I expect it to be within the 3-4" that these rifles are known for with service ammo though. I tried the same load without the gas check just to see what would happen and it opened the groups up to about 3 inches at 25m, so if anyone is considering using this load don't skip the gas checks. The load also cycled the gun properly, which was impossible with the 7383 powder with the lighter bullets I was using before. So now I have a nice 11 cent per round full power training load for the CETME, which is nice.

[runfiverun]

hunnh.
who'da thunk it was possible...

[Scharfschuetze]

Well done!

[dogmower]

I use 25 grains of h4895 with the lee 165 (what mine comes out as) tl spire point bullet. shoots to point of aim at 200m, cycles the action, and doesn't beat up the cases too badly (for an hk).