Tough one to figure out

[belizejet]

I have a BHW 18" barrel in 6mmx45. Last year I had it built up on an upper and loaded a ton of different ammo with it. I did a lot of testing. I was getting near MOA groups with 70grn nosler BT varmint loads behind H-335. I loaded about 100 of them and was preparing to use it for varmint hunting. I then decided that I wanted to get the barrel threaded because it was just target crowned. I took the upper apart and sent in the barrel and a silencerco QD muzzle break so I could use my suppressor on it. Got it back and it looked great. Built the upper back again and took it to the range to sight it back in with the can on it. I decided to use the same ammo that was giving me pretty good result from before. This was from the same batch I had loaded before about a year ago. I couldn't get the thing to hit paper at 50 yards after bore sighting. I finally moved up to 25 yards. The bullets were hitting the paper sideways!!! I took the can off thinking that we effecting it. With just the brake on it it was still tumbling bullets at 25 yards!! Same rounds I was getting 2.5 in groups with 200 yards was now not even MOA at 5 yards. I checked the brake and it seems aligned. I'm stumped. Next step is to take off the brake and shoot it without. I ran out of time before I could do that. I'm guessing either I built the upper up wrong or the barrel got screwed up at the smith who threaded it. Any thoughts? I've built a bunch of uppers (10 or 11) and never have seen this before.

[Great-Kazoo]

Possibly the crown. OR................ the brake is just tight enough (diameter wise) it's throwing your shots off. Have you measured the clearance? I had a similar issue with YHM brake. Threw it on the lathe, opened it up to the next size and no problems.

[BPTactical]

I would check the threads for concentricity and the brake for same. Pull the brake and check for any evidence of strikes.
Unless you have a well practiced eyeball for anomalies such as this your eye won't tell you much.
A suppressor alignment rod or range rod would be a more accurate method.
I can check it if you like.
Nothing you did as far as a build would cause keyholes to the best of my knowledge.
I would say you sent the barrel to the wrong Smith.

[ray1970]

I would say you sent the barrel to the wrong Smith.

I agree.

[belizejet]

It could be the brake being too close in tolerance. It's a .223 brake from silencerco which is measured at .25. I was shooting 6mm (.243) through it. Not strikes on the brake so maybe it just needs a few more thousandths.

[davsel]

Only time I've had that happen is with a poorly threaded barrel that caused hits to the end of my brake, 3" from the end of the barrel.
I opened up the brake and it shoots fine. Can't ever put a suppressor on it though.

Since it's an 18" barrel, you can have it cut back and re-threaded - by a different gunsmith.

[eneranch]

It could be the brake being too close in tolerance. It's a .223 brake from silencerco which is measured at .25. I was shooting 6mm (.243) through it. Not strikes on the brake so maybe it just needs a few more thousandths.

Probably should have led with that.

One would think a qualified gunsmith should have caught this.

[belizejet]

Really? I thought it would perfectly. Live and learn I guess.

[eneranch]

Once you get it fixed, let me know how it shoots.

[asmo]

You changed the barrel harmonics. That means you need to develop a new load.

Nor saying anything said above is inaccurate, just saying you need to do load development whenever you change the size, weight, characteristics of the barrel.