220gr SMKs not stabilizing.

[MadDog]

Well, I tried some more loads. Some 220gr SMKs at around 1100 fps, Some 220gr round nose at about 1025, and some 240gr SMKs at around 1140 fps. None were stabilized at 100 yards. I did try some 190gr SMKs at around 1020fps that did stabilize and the accuracy was much better. I still don't know why the longer bullets will not stabilize out of my 1 in 8 twist barrel.

One issue that I noticed that may or may not be related. When I cleaned the upper the first time, I noticed a lot of small copper particles on the bolt and inside the carrier. This time when I cleaned it, there were no particles of copper but there is a considerable build up of copper on the bolt and inside the carrier. I will attach some pictures below. The barrel itself is cleaning up very easily with very little copper.

I have never seen that type of build up on my 223 uppers. Is this common with subsonic loads? I am shooting suppressed if that matters.

Any ideas?






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Looking for some advice- what would you do next?

I purchased a new 300 Whisper upper several months ago and just finally found time to start loading for it. After reading my manuals and posts here and on other sites, I decided on a couple of starting loads. My goal was to get 220 gr SMKs as fast as possible but still be subsonic. My barrel is 10" and has 1 in 8 twist.

I took the rifle out today and tried out several loads. None of them stabilized very well. I was pretty disapointed. I really thought that the 1 in 8 twist barrel would handle the 220gr SMKs without any problem. I will add a couple pictures of targets that include the load and speed of the rounds fired. As you can see the accuracy was pretty poor also. Since I am using a suppressor, I tried some of these at short range without the suppressor and they appeared stable at about 10 yards. Also I tried some at 100 yards without the suppressor and they were the same as the groups pictured.

Shouldn't they be stable at these speeds in a 1 in 8 twist? I could go a little faster but they were pretty close to being super sonic already.

Any suggestions besides trying different bullets?

Thanks,

Joe

[jjmcrowell]

You might try and make sure it's really a 1:8 barrel. Perhaps take a cleaning rod with a tight fitting patch, insert it and mark the position with a marker on the rod. Then slowly push it through the barrel, noting the distance till the muzzle and how many times it rotates. Just guessing, but if it's a 10" barrel and the chamber is 1.4" long it should have rifling a little over 8" long...this should give you 1 complete rotation of the cleaning rod. If you get less than one rotation, perhaps it's a 1:10 barrel.

I think a 1:8 should have no trouble with 220gr bullets.

[MadDog]

Originally Posted by jjmcrowell

You might try and make sure it's really a 1:8 barrel. Perhaps take a cleaning rod with a tight fitting patch, insert it and mark the position with a marker on the rod. Then slowly push it through the barrel, noting the distance till the muzzle and how many times it rotates. Just guessing, but if it's a 10" barrel and the chamber is 1.4" long it should have rifling a little over 8" long...this should give you 1 complete rotation of the cleaning rod. If you get less than one rotation, perhaps it's a 1:10 barrel.

I think a 1:8 should have no trouble with 220gr bullets.

Man, that is the first thing I did when I got home from the range. I thought the same thing- some one screwed up on the barrel. I did just what you described, twice. It is 1 in 8 for sure.

Any idea why it would not stabilize those bullets at those speeds?

Thanks,

Joe

[THE DOCTOR]

i do not know I have a 1/8 and i have no trouble stabilizing 240's so the 220 for you should be no big deal, but I am running a 16" barrel so that might make a difference too. You might try lapping the bore. I have heard good things about that.

[Pitt300]

Whose barrel is it?

[JFettig]

How does the muzzle look? If the crown is off it could be throwing the bullet off.

like pitt300 said, who made the barrel?

Jon

[MadDog]

Originally Posted by JFettig

How does the muzzle look? If the crown is off it could be throwing the bullet off.

like pitt300 said, who made the barrel?

Jon

The crown looks good.

I bought the upper complete from SSK. I was trying to not name them until I had a chance to talk to them but I guess there is no harm. I just didn't want start a bashing thread. If it turns out to be a problem with the upper, I am sure that they will take care of it. The barrel is a heavy, free floated, stainless, "match grade" barrel.

The upper has an adjustable gas system and has cycled with all the loads that I have tried including Corbon and some SSK loads. All the ammo I have loaded and the Corbon and SSK ammo all used the 220gr SMKs. I will probably try some other bullets / loads to see what happens before I talk to SSK.

One thing that I did notice when I cleaned the upper today was what appeared to be some small pieces of copper on the bolt and inside the carrier. I have never seen that in any other AR15 type uppers before. At least not to that extent. It made me wonder if there was a burr or some other problem where the gas port enters the barrel. Could that cause enough of a problem to effect the stability of the bullets? Just looking down the barrel, I can't see any problem.

I put about 40 rounds through it today and I have run about 60 rounds total.

Joe

[pomofo]

Could there be a small variation in rifling twist, say 1:8.5" or something? There is often some rifling variation in the production process, with some 1:8 barrels being under 1:8 and some being over. The shorter the barrel, the more important it is to get as fast a twist as possible to stabilize long, heavy bullets. If I remember correctly, the 5.45 Krinkovs have a 1:6.3" twist, as opposed to 1:8" for the normal AK-74, in order to stabilize the long 5.45 bullet. If your twist is just slightly over 1:8 I would think that might be enough to destabilize the heavy bullets. If you can stabilize lighter bullets then I think you could definitely point to a twist problem.

[JFettig]

So the crown looks concentric, correct? I think the best thing would be to contact them.

Did you test corbon or ssk loads to see if they stabilize?

btw, lapping your barrel is a last resort, its something that has fixed M1S barrels but is probably something that should be done before the barrel is chambered(what a friend told me). It'll definitely void any warrenty you might have.

Jon

[320pf]

Originally Posted by MadDog

The crown looks good.

I bought the upper complete from SSK. I was trying to not name them until I had a chance to talk to them but I guess there is no harm. I just didn't want start a bashing thread. If it turns out to be a problem with the upper, I am sure that they will take care of it. The barrel is a heavy, free floated, stainless, "match grade" barrel.

The upper has an adjustable gas system and has cycled with all the loads that I have tried including Corbon and some SSK loads. All the ammo I have loaded and the Corbon and SSK ammo all used the 220gr SMKs. I will probably try some other bullets / loads to see what happens before I talk to SSK.

One thing that I did notice when I cleaned the upper today was what appeared to be some small pieces of copper on the bolt and inside the carrier. I have never seen that in any other AR15 type uppers before. At least not to that extent. It made me wonder if there was a burr or some other problem where the gas port enters the barrel. Could that cause enough of a problem to effect the stability of the bullets? Just looking down the barrel, I can't see any problem.

I put about 40 rounds through it today and I have run about 60 rounds total.

Joe

Joe,

Welcome to the dark art of shooting long heavy sub-sonic bullets. I have had the same thing happen to me with speer 165 g boat tail bullets. The bullets were seconds so I think that part on the problem was that the the bullets may not have been perfectly concentric and/or there were some pockets in the lead core.

With the bullets loaded to 1000 fps, I could get groups of about ~1-1.5b inch at 100 yrds. They would "key-hole" the targets just like your photos show. However, once I got the velocities up to about 1080, they shot better but would "key-hole" still every once in a while.

A long a similar line, another 300-221 fireball shooter was having the same problem with 240 g Sierra MK but his loads were clocking about 980-990 fps. I suggested that he increase the velocity. The stability improved the faster he pushed the bullet. Once he got his loads up to about 1080 fps things were ok.

I think that there may be a couple of different things going on. If it is a brand new barrel you might still be in the break-in period. That would explain the excess copper that you are seeing inside the bolt and carrier. You are working out the rough spots etc... I would try cleaning the barrel and see if things improve.

In addition, I think that the long bullets are "critically" stabilized at these subsonic velocities so it does not take much to start them tumbling. This tendency to tumble is what makes the sub-sonic load effective... When the bullets hits is starts to tumble straight away and really tears things up.

So I would also try increasing the load a bit to 1080-1100 fps and see if that helps. The speed of sound at 50 deg F is 1107. At 70 deg F the speed of sound increases to about 1128 fps.

Good Luck I hope this helps

320pf