I have a PRI Mk12 mod H that has suddenly lost its accuracy. It held 1moa 10 shot groups all day with my handloads. It has suddenly opened up to 3moa. I checked torque in the optic and rails, tried shooting without the suppressor, and decoppered the barrel. Still shooting 3 moa. The rifle has about 1500rnds on it so it’s not shot out. I ran the handloads through a friends rifle and it grouped fine so ammo doesn’t seem to be the culprit. I took the brake off and cleaned the crown very well and noticed a scratch very near and into the muzzle. I am not sure if this is enough to cause the accuracy issues. The pictures attached do make the scratch look much more severe due to the light. I couldn’t get it to show up at all without outside light so keep that in mind. My next step was to swap scopes to make sure it’s not the optic. I’m trying to get through the last of my hand loaded 308s before I swap (changing bullets). I don’t want to re-zero with such a low round count left.
Gunsmithing Crown Damage
- Thread starter IeatRocks
- Start date
IMHO, it looks very possible you damaged the business end of the crown , just based on the trajectory of that scratch/damage trail.
I'd want to take a closer Look at the actual land/grooves at the crown, in some good light. Maybe use a camera on a tripod with zoom/macro settings if this helps to see it.
Also, I'd guess some more background would probably useful/helpful as do how you damaged it. Just event or object or what exactly caused that scratch?
I'd want to take a closer Look at the actual land/grooves at the crown, in some good light. Maybe use a camera on a tripod with zoom/macro settings if this helps to see it.
Also, I'd guess some more background would probably useful/helpful as do how you damaged it. Just event or object or what exactly caused that scratch?
I will try to get some better pictures with my real camera. My iPhone doesn’t have a macro setting.
I honestly have no clue how I scratched it. The gun gets ran hard. It’s been knocked over a couple times, banged into things, etc. but nothing I’ve noticed directly to the muzzle. The first year of its life it only had the brake, no suppressor, so it is entirely possible that something did make its way in there at one point.
If it is the crown how tough is it to diy repair it? I’ve found some tutorials online that make it seem easy, however, the tutorials are guys cutting the barrel down and then recrowning it vs just a repair.
I honestly have no clue how I scratched it. The gun gets ran hard. It’s been knocked over a couple times, banged into things, etc. but nothing I’ve noticed directly to the muzzle. The first year of its life it only had the brake, no suppressor, so it is entirely possible that something did make its way in there at one point.
If it is the crown how tough is it to diy repair it? I’ve found some tutorials online that make it seem easy, however, the tutorials are guys cutting the barrel down and then recrowning it vs just a repair.
If you want to give it a try, hit it with reloading inside the neck chamfer tool. Cleaned up a few milsups this way before.
Forgive my ignorance, are you suggestions taking a neck chamfer tool (for brass) to the muzzle to clean it up?
I dinged the crown of a Rem 700 I have. Used a 45 deg. de-burring tool to fix it. Very light pressure, basically the weight of the tool, and a few careful turns and all is well. This is what I would do again were that my rifle but I am not saying or recommending that is what you should do. A new crown by a smith is always a good thing......
If that scratch is not as big as it appears in the pics, as you said, then it's not a big deal at all to remedy. However, the proper tools are sort of expensive. A shop won't charge you much to take care of that crown.
Personally, I'd give it a whirl myself (using improvised tools), nothing to lose...you can still take it to a shop. Valve grinding compound on a marble? I can think of a few other methods.
Personally, I'd give it a whirl myself (using improvised tools), nothing to lose...you can still take it to a shop. Valve grinding compound on a marble? I can think of a few other methods.
Getting a bit better light on it. It does appear there is maybe just a bit of damage to the inside lip of the muzzle. I like the idea of lapping compound and a marble, seems simple enough. If it doesn’t fix it up I can take it to a smith. I don’t have one close to me so if I can save myself the drive it is preferable. Open to any other opinions and thank you to everyone so far.
Meant to mention, in the barrel there are some piece of lint from a rag. Just an FYI as it almost looks damaged.
Meant to mention, in the barrel there are some piece of lint from a rag. Just an FYI as it almost looks damaged.
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You might try a ball bearing instead of a marble. Either should work but it's not going to be a fast thing and you don't want it to be. Get some 800 grit valve grinding compound and just spin the round thing without much pressure. If you apply pressure, you run the risk of not applying it evenly...I know that sounds counter-intuitive, it's round and SHOULD apply evenly. Just very very light pressure while spinning. I have thought about this a bit since you first posted...I thought about using hot melt glue to glue a dowel rod to the round thing so you have a nice way to spin it. Just a thought.
Round head Phillips screw in your cordless drill and valve lapping compound.
Curious, how would a high speed steel or carbide cutter not allow any cuttings into the bore? I'm no machinist.
I would not plug anything with anything.
Obviously, just clean thoroughly as per normal, breech to muzzle, before sending a round.
Because there's a difference between cutting abrasives and material cuttings.
Yeah...whatever.
I would not do many of the other things mentioned so far but I absolutely would not be worried about some stray abrasives from the valve grinding compound any more than I would be concerned with metal shavings. It's not like anyone would be so stupid as to fire the rifle before cleaning thoroughly. I'd simply start by hosing the bore with an aerosol blast of something like even brake or carb cleaner. Then I'd clean as per normal and go see if the efforts had paid any dividends.
I would not do many of the other things mentioned so far but I absolutely would not be worried about some stray abrasives from the valve grinding compound any more than I would be concerned with metal shavings. It's not like anyone would be so stupid as to fire the rifle before cleaning thoroughly. I'd simply start by hosing the bore with an aerosol blast of something like even brake or carb cleaner. Then I'd clean as per normal and go see if the efforts had paid any dividends.
Do you guys actually think this is causing performance issues and really needs to be redone?
From OP, the rifle was shooting much better than now. The barrel has a low round count. The crown damage is what he has it narrowed down to.
If there is a scratch that the bullet is rubbing on as it exits, absolutely a scratch can create issues. Particularly a scratch perpendicular to the rest of the bore. I seriously doubt the scratches that we see on the surface of the crown cause much problem at all but the line crosses into the bore itself.
Recrowning, even having a shop do it, is not very expensive and surely worth the effort.
EDIT: the scratch or damage might "cure itself" with enough copper jacketed high speed honing devices sent across it.
No, I don't. I think removing the copper fouling will help much more than cutting a new crown. You can see the copper looking into the bore.
I de-coppered the barrel in my initial process of elimination. The gun shot equally as poor even after it started to foul up again.
This.
That minuscule scratch across the crown is now way going to affect the flow of gas across the crown to deflect the bullet.
Check the edge of the bore with a Q-tip to be sure it doesn't grab any fibers. I always put a very small 60 degree counterbore at the edge of the crown after cutting to eliminate any sharp burrs/edges.
The lateral scratch at the 3 o'clock position appears to be the main issue, as it crosses the land. You have a few options to address this. One option is to use brass crown laps from various suppliers to try lapping the scratch out by hand as it appears to be a very minimal depth. A bore scope would reveal the true nature of the damage, not everyone has access to one though.
Brownell's offers crown cutting tools that can be effective, depending on your level of patience—patience = precision. However, they may not have a tool that would match the specific profile on your crown, you would have to look. I’ve had good results repairing crowns using hand tools, though I agree that nothing compares to a lathe-cut crown. One member suggested having a gunsmith recut the crown, but this may not be feasible for you in the short term, and I totally understand if that is the case. If you have the tools or want to own them, recutting and lapping by hand could be a viable solution and save you some $$. If you're not satisfied with the results, you can always seek a gunsmith's help, but resolving the issue at home is also a valid option.
That wee bitty scratch ain't doing shit.
Clean the damn bore till it's bright and shiny and not slammed with copper fouling.
It's so fouled it's hard to make out any lands and/or grooves.
People are always looking for some damn excuse to get around proper maintenance......
Always.
Clean your damn rifle.....correctly this time.
Proof.....shag nasty.
Clean the damn bore till it's bright and shiny and not slammed with copper fouling.
It's so fouled it's hard to make out any lands and/or grooves.
People are always looking for some damn excuse to get around proper maintenance......
Always.
Clean your damn rifle.....correctly this time.
Proof.....shag nasty.
What solvent did you use? Also, I am sure you may know this, but after cleaning all the copper, you need to re-fowl the bore. All my rifles differ, but after a copper treatment it takes on average 15-25 rounds depending on which gun, before the groups tighten up and I get consistency.
I used Sweets. The bore was mirrored after cleaning. It still shot poorly after even after fouling. I haven't had the time to get to the range to confirm if the crown work has improved anything. I used a large ball bearing and lapping compound and stepped up the grit as I went. I didn't go too crazy with it, just a lot of time a lot of spinning.
This thread reminded me about an article I read a while back. Based on the testing in the article, I would probably look beyond the crown.
rifleshooter.com
Do rifle crowns matter? Does a crown affect accuracy? I find out with the help of my grinder…
Do rifle crowns matter? Does a crown affect accuracy? I think most of use would say yes. It seems intuitive that the crown, the last part of the rifle that touches the bullet before it leaves the…
rifleshooter.com
Sorry for the late update boys. Really been scratching my head with this one. I shot the rifle after polishing the crown, still shot poorly. Maybe a little better than before but nothing to really note. I decided it was time to take it to the smith. He went over the entire rifle and was not able to find anything wrong. The crown and barrel looked great. I bought two separate boxes of match grade factory ammo and I am getting similar results as my hand loads just to rule out any of my reloading skills. The only thing I have yet to change out or test is the bolt and BCG. Not sure where to go from here really.
Have you tried shooting it without the muzzle device? I went through a similar issue years ago where I had a configuration that was shooting lights out until one day it wasn't. I was getting ready for a match and my groups opened up big time and it was very frustrating and disheartening because I couldn't figure it out. I went through the same process of elimination and but nothing really shook out. So close to the match date I decided to replace the barrel but I had issues all throughout the match so while doing more troubleshooting I took the muzzle device off and happened to look inside. There were a couple large gouges at the muzzle aperture that I surmised were the cause of my grief because when I swapped out muzzle devices accuracy went back to acceptable levels. It could be worth checking the muzzle device in this case also, just as another potential item to check off.
I have tried shooting it without. No improvement. The smith took a look at the brake as well and said powder burn looked fine and couldn’t find anything after cleaning either.
I’d like to know too. But correct, the smith found zero issues with the barrel or crown. Zero issues with the rifle period.
I went through a similar situation with an AR upper recently. It was shooting great then suddenly 2-3 moa no matter what I tried.
I fixed my issue by replacing my gas tube. I know that sounds dumb but that’s what I did. I think my gas tube somehow got bent such that it was binding slightly when the carrier went into battery.
I fixed my issue by replacing my gas tube. I know that sounds dumb but that’s what I did. I think my gas tube somehow got bent such that it was binding slightly when the carrier went into battery.
I think of this whenever something thinks the crown is causing accuracy issues. Years ago I working in a gun shop. There was an old guy who always cut his rifle barrels off with a hack saw (legal length). He did nothing to clean up the crown or end of barrel. That all shot as good or better than before he cut them off. It baffled me back then but that article reminded me about that old guy and his hacksaw.This thread reminded me about an article I read a while back. Based on the testing in the article, I would probably look beyond the crown.
Do rifle crowns matter? Does a crown affect accuracy? I find out with the help of my grinder…
Do rifle crowns matter? Does a crown affect accuracy? I think most of use would say yes. It seems intuitive that the crown, the last part of the rifle that touches the bullet before it leaves the…
Ran the BCG without the bolt, felt a little sticky. Worked the tube and block to change the alignment and got it feeling much smoother. I’ll get it to the range and see what happens. Thanks for the tip.
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