[B]LAST UPDATE[/B] 16X Mk4 & borescope pics <800 rounds sudden loss of precision

[308sako]

Short story; barrel is a 16" chrome lined from a decent manufacturer/sales unit. Initial results were consistent true sub MOA for 5 shot groups. Changed clean muzzle direct thread suppressor to an AAC BrakeOut, and SR-5 suppressor. Other change was from ACOG to S&B 1.5 x 6. Inspection of crown shows no damage awaiting a friends bore scope evaluation. Rifle is now a 3 MOA POS.

These changes were reversed but inaccuracy remains.

My gut feeling is somewhere in the pipe some chrome came loose.

A fellow shooter suggested that maybe the gas port is an issue.

Any suggestions before yet another tomatoe stake finds it's premature way to the garden?







Interesting occlusion, so other than the lands starting at irregular intervals, and the GP showing normal erosion, the crown passable, the little occlusion in that land... what could happen. Still leaning to trying a better optic and the remaining ammunition previously proven.



Firing was done clockwise after getting a basic zero (4) rounds of fouling. Round #4 jumped out and #5 was back in. Came one additional click to the left and the first 3 in second group were together, then not :-) Last five above were fired at a quicker pace to alleviate some of the variable wind conditions... Blustery 8 - 12 MPH, squirrely conditions. FWIW, my 11.5 with ACOG 4X shot better with 3,700 rounds logged. Lower is a old Colt with a Jewell trigger that breaks better than my Giessle Hi-Speed match's.



My thought is you lightly kroil the bore and see what that missing chunk area looks like next.



Range day 5 13 21, 93F 10%H 12 - 15 MPH @ Desert Sportsman's club Las Vegas, range #9, distance to target: 100 yards:I can live with this level of "precision. Not what was but not unacceptable either.

52 grain Berger FBHP and VV 130 Velocity 2963 SD 13.2/ES 29

62 grain Hornady BTHP and BenchMark velocity 2960 SD 17/ES 35

[Spooky1]

I am just pulling this outta the air, could the barrel nut torque have loosened some throughout your shooting. Then installing the muzzle device sorta through everything outta kilter?

This is sorta me thinking outloud, I hope you get it figured out and it not be a pain in the ass or your wallet.

[Ned Christiansen]

Overtorqued the second muzzle device so bad it collapsed the bore to the point of it taking damage that was not undone by removing the device. Gage pin the muzzle and see if it will take like a .219 or .220 but only for the first 1/4" or so.

I actually doubt you did that but..... could be.

[mRad]

Overtorqued the second muzzle device so bad it collapsed the bore to the point of it taking damage that was not undone by removing the device. Gage pin the muzzle and see if it will take like a .219 or .220 but only for the first 1/4" or so.

I actually doubt you did that but..... could be.

This was my initial thought. Rare as it may be, if it changed with muzzle devices.


Another possibility would be ammunition. Has the lot changed? Is it possible in the process of changing the muzzle device enough torque was applied to cause a problem with loosening the barrel nut?

In my experience the manufacturers considers 3 MOA “in-spec” even for precision barrels. I’ve seen it with a brand very reputable around here.


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[georgeib]

Another possibility is the optic mount. Make sure it's tight. Use witness marks, if you don't already.

[Stickman]

Short story; barrel is a 16" chrome lined from a decent manufacturer/sales unit. Initial results were consistent true sub MOA for 5 shot groups. Changed clean muzzle direct thread suppressor to an AAC BrakeOut, and SR-5 suppressor. Other change was from ACOG to S&B 1.5 x 6. Inspection of crown shows no damage awaiting a friends bore scope evaluation. Rifle is now a 3 MOA POS.

These changes were reversed but inaccuracy remains.

My gut feeling is somewhere in the pipe some chrome came loose.

A fellow shooter suggested that maybe the gas port is an issue.

Any suggestions before yet another tomatoe stake finds it's premature way to the garden?

Who assembled the URG when brand new?

Have you removed the barrel and checked the barrel extension fit to the receiver?

Who makes the barrel? Clearly the barrel is not the issue.

What ammo yielded submoa groups? At what distances? How many rounds fired per grouping?

How was the brake installed?

Has it been fired brake only?

Has the optic been verified on another known good weapon?

What mount are you using?

Have ALL bolts been tightened?

These are just a few questions while grabbing coffee....

[308sako]

NedC & MRad; on the over torque issue I think not at 25 ft/lbs.

MRad; ammunition has not changed, same lot as previous. My omission was that the loss was not immediate after the muzzle device change.

GeoB & Stick; the mount is an Aero Precision, screws tightened to spec.

Stick; I assembled this upper using a Giessle action rod. I've not fully taken it down as yet. I am not blaming the "manufacturer" which is in this case BCM. Ammunition consists of handloads using Sierra 60 grain HP, distance to target 100 yards from benchrest, groups fired SLOWFIRE in 5 shot strings while chronograph in using a LabRadar. No anomalies noted. The scope has not yet been tried on a known good rifle.


Gentlemen tank you for your input and direction. I shall update.

[grizzman]

It seems like there was no change made at the exact time that the accuracy was lost.

Have you thoroughly cleaned the barrel since groups went to shit? Although, if it was <1 MOA one day a d 3 the next, copper fouling is unlikely to be the reason. My semi-informed opinion lays the name on the optic or mount.

[markm]

I'd switch back to the COG or any other aiming device. The only time we had it NOT be optic related was on a crappy Faxon barrel that was trash out of the gate.

[C-grunt]

I had a similar situation years ago. Had a BCM SS410 barrel that was shooting consistently .75-1 MOA then one day it went to 2+ MOA. I couldnt figure it out and had IraqGunz look at it. He noticed right away that the shop I bought my suppressor from had used a crush washer instead of a standard type washer when installing my mount. It lasted for a bit but eventually started becoming a problem. As soon as he put a regular washer on it the rifle returned to normal.