I have a bull barrel 308 rifle that had a 21” barrel. I just had it cut to 17” and threaded. Prior to this it would shoot 1” or less groups all day long. Got it back and shooting it with a can on I can’t get better than 6” groups at 100 yards. Everything is tight. Could the cutting and threading have mattered this much or is something else wrong?

How did it shoot after cutting, without the can?

Pretty wild. Something is out of square.

May be bad crown job. Probably needs totally different load work up than before.

Didn’t try it. Was excited to try can and just went through about fifty rounds trying to figure out what the heck.

I was planning on taking it to go shoot a pig in the morning but am taking ol faithful instead.

It shot good with anything I put in it before. I was shooting 165gr game king from custom reloads of Dallas today.

Try it with a thread protector, or a brake. 3-4 rounds should tell you something.

Try without the can. could be the threading was done poorly, could be the suppressor causing the issue. only one way to tell between them

So, it was shooting a 21" barrel and no muzzle device inside 1" ? Correct?

Cut to 17", add suppressor, same ammo, and now it is shooting 6" groups. Is that right?

If the threads are out of center, you would have struck a baffle already. Have you looked down the suppressor yet?

Cutting threads on-a barrel definitely changes the last 1/2” of your barrel you can’t move metal on the outside without moving it on the inside. This is especially the case with small rimfire barrels.

Im thinking you had a baffle strike like fireman suggested, and whomever cut your threads failed to indicate that bore in before cutting the threads.

Correct. I have not looked down can yet. Busy loading trailer for trip to ranch tomorrow. I will be there all day tomorrow and report back tomorrow night.

Cut and thread was done by class three machining. Not saying a mistake is impossible it I think they are pretty highly rated for doing good work?

Cutting a barrel most certainly can affect accuracy, but not so much because the barrel was cut, but more so along the lines of barrel/ammo synergy. Clint Smith did an article about 20 years ago where they cut a .308 barrel down 1" at a time from 22 or 24" to 16" and shot for groups at 100 yards using the same ammo. They used the same gunsmith to cut and recrown the barrel each time. Interestingly, the group sizes varied in something of a sine wave based on the length on that particular rifle with that particular ammo. So the two best groups came from about 3" different in barrel length.

Later the found that tweaking the ammo load could produce excellent results at virtually all of the barrel lengths, something many reloaders know and love to do. So it came down to matching loads, lengths, and harmonics to find the ideal for each length.

I would suggest to Sewer Rat that he test the barrel without the can and see how it performs. Adding a muzzle device can also have a significant impact on how a given barrel performs with a given load, but his numbers do seem a bit extreme. I have see a 1" grouping barrel (with a particular load) go to 4" group size and be 7 inches off to the side by adding a can. Removing the can produced the original grouping again.

Hopefully, testing without the can will reveal the barrel is still sound, or maybe it will reveal that it was crowned improper. Maybe the threading is off as stated. Hopefully, it doesn't just hate the can.

Tuning applies to everything that carries harmonic waves, from radio antennas to musical instruments and speaker enclosures.

Was the next place I was going.

Thread job is probably fine. A suppressor is a significant weight hanging off the end of the barrel.

For everyone that drops off a rifle to me for load development, I have to shoot it with the exact muzzle device that the rifle will shoot with full time. This rifle in question, just might not like the original ammo anymore. Most specifically the powder charge. Chad makes really good ammo, and he has tested his for sale ammo in a variety of rifles. But none of us can test seating depths and charges in every possible combination and come up with a charge that shoots well in every combo.

I have. 300 Win Mag ammo out there with the same brass, primer, powder, and bullet. All have a different CBTO, and all have a different charge. Some very by a few tenths grain from one to the next, but the overall spread is literally 5 gr of powder. That is due to the large variety of barrel lengths, contours, free-bore and muzzle devices I have had combinations of, across the board on all the .300 Win Mags.

Very interesting. I have two different 30 cal cans I will try it without and then if it shoots good try it with both cans and see if one does better than the other.

I had to take the scope off to send it in for threading and I put it on the same the best I could and it was shooting 8” right of where it was before I disassembled and sent it in.

I didn’t get to mess with it anymore this evening. I was out past dark loading up all this.



It is a rake for my tracked skid steer and a weld on rake for my dozer. I have been grubbing brush at the ranch with the excavator and need to rake and stack it. Plan is to go shoot some pigs in the morning and then weld this rake on and work on some brush tomorrow.

Wait wait wait! Was the scope change at the same time as the barrel cut? If so you may be looking at the wrong demon. If not, carry on.

If the threading was cut out of round, it will degrade accuracy. I've seen this personally on my 308. Mine would shoot about 7" to 9" groups with the can on. I took the can off and it would shoot sub 1" with my standard 155 grain ELDM bulk ammo. I put the can back on, and big groups. I took it to my gunsmith, and he said the threading was so out of round, it pegged his gauges. He chopped 1/2" off, and rethreaded the barrel. It shoots great now. Who threaded it?

Everyone has posted great info.

Will add

1. every torque setting was changed when took it apart and reassembled. Scope, stock etc.
2. New load will likely need to be developed, if its 1/12 twist are even more sensitive to shortening
3. Make sure mount is torqued correctly and not a pile of peel washers were needed to time it. Guy on here thought his was messed up but had like .200 worth of peel washers on it. Once we reduced it to 2-3 his accuracy issues stopped.

Confirm that mount and can are in spec, see 10-15 a month that arent. All cans have runout, most in the .015-.020 range. Had an arfcom guy report weird accuracy after threading. He came over and we ran some test and found out can had .040 runout. He sent it back to them and all is good now.

Few rare guns just hate being suppressed. Had a guy have issues with factory threaded one, had issues with another shop redoing it, I did it and it passed all of mine and his alignment tests and still didn't like a can on it.

They are all alignment checked prior to boxing them up.



Do these on a cnc mandrel between centers most of the time for many years now, takes virtually all human error out of it. Only error that can be introduced is tailstock, which would just be a slight taper but still concentric.

Having said that I stand behind my work and if it can't be fixed I will buy it, have it rebarreled etc,

Luckily in 13 years now 40,000 plus only had to buy 2.

Shoot me an email if can't sort it out and we will go from there.

2/40,000. That is impressive.

More than that, the response is impressive. You do great work Morgan, thank you for the extra mile you go as well

Thanks man, that means a lot

With this being said, it seems we only got 50% of the story as the optic was messed with as well.
I bet the optic, or mount has some issue that is not standing out and as Chris said he’s chasing the wrong demon.

Sounds like your process for threads is well proven and kudos to you for coming on this thread and explaining how you do your work.

Shoot it without the can first. That will at least narrow it to the can or threads/muzzle. If it shoots check for baffle strikes, it could be as minor as a glint on one baffle. If you don't see any, you might have to have a reputable smith throw it in a lathe and dual indicate it to see if the muzzle and threads are concentric & in line to the bore, not the exterior of the barrel. Even if it clears all the baffles, it could cause problems if it's getting close to one side and not traveling down the center of them. I've seen it multiple times. Same with brakes.