Suggestions Needed - Rifle Fell Off the Accuracy Cliff

[ShadH]

I’ve been a lurker here and hate for my first post to be one asking for assistance, but I need some help brainstorming from some long range folks.

I have a Browning Hells Canyon Long Range McMillan in 300wsm topped with Nightforce NX8 on Nightforce rings. I love the fit, feel, and finish of this gun.

It’s got ~300rds through it. I bought it new and did a proper barrel break in. It was a little finicky with handloads, especially around seating depth, but once I got it dialed I was consistently getting 1/2” MOA groups at 100yds and some even better. 3-4” groups at 500 and I was 8/10 on a 8” target at 940yds. It was an absolute shooter primarily with 190 gn VLDs. Then something happened and I cannot figure out what.

It started when I would get flyers here and there every 5th shot or so. Assumed it was me. Then groups just started rapidly deteriorating and within the last 80 rounds it got to the point where it’s 2-3 MOA over 5 shots. It’ll shoot some in the same raged hole and some all over the place. It’s not usually stringing- but I’ve notice a slight trend in that the flyers tend to be low left, but sometimes high and low some left and right. I switched bullets just to see. Shot 3 180 accubonds the other day that were literally through the same hole. Let the rifle cool for a few minutes and shot two more. Both low left about 3/4” apart and 1 1/2” away from the other group. WTH?! I came back the next day and shot the same load. No group at all, random holes around 4”.

I’ve taken everything off. Checked my bases, rings, and screws, retorqued. I’ve checked the stock bedding and ensured free float. I’ve retorqued the stock bolts. I’ve stripped the bolt to make sure nothing is wrong. I took it to a gunsmith yesterday to have a bore scope through it. No carbon rings, throat and barrel look great. I tried different handloads and the loads that I was previously dynamite with. Thinking it was me I set this gun up and my Kimber Montana in the same caliber and shot it lights out followed by the same random grouping in the Hells Canyon. The next weekend the 300 RUM shot excellent as well so I assume it’s not me. I’m lucky enough to have a shooting range that’s 30 feet away from my reloading station so I have the opportunity to really adjust things and check for improvement but I’m at a loss.

The more I shoot it the worse the groups are getting. Has anyone ever had this before? Any suggestions at all? I hate to part with what was at one point the best shooting rifle I’ve ever had if there’s something I’m missing, but I just can’t think of what else it could be.

 

[ShadH]

I should highlight that I’ve cleaned the heck out of it to ensure no copper fowling- bore scope confirmed there are no copper rings or deposits.

 

[Mike Casselton]

Next step is to try a different and trusted scope

 

[Hawk in WY]

You might drop the load a grain or two.

Summer temperatures might have pushed you off the node.

Good luck.

 

[OREGUN]

Any idea of the before and after muzzle velocities? Could be related to the barrel speeding up. When you say it was finicky when you were doing load development, was it anywhere near this bad when the load was wrong? Could the barrel have sped up AND the throat eroded a little to result in the load both no longer being the right charge and the right seating depth? It would surprise me if any of this would cause the groups you are seeing.

I think a known good scope would be a good place to start. It sounds like a loose base/ring screw or other fastener…but if those are all checked….an effed up scope seems likely.

 

[celltech]

You keep referring to copper, but what about carbon buildup? Let's see a scope pic from the throat...

 

[ShadH]

You keep referring to copper, but what about carbon buildup? Let's see a scope pic from the throat...

I didn’t look myself but the gunsmith said no carbon or copper build up. No signs of throat erosion. Chamber, barrel and crown were pretty much immaculate. I didn’t look myself or take pictures.

 

[sgtsmmiii]

You keep referring to copper, but what about carbon buildup? Let's see a scope pic from the throat...

My thoughts, too. A buddy's 6.5 Creed did the same thing. We checked it with a Lyman bore scope - and lo and behold - the dreaded carbon ring right at the start of the throat. Cleaned it away with Iosso and it regained it's original accuracy.

But, the other suggestions posted all deserve merit. Try to check 1 variable at a time so you have an idea what caused it when you get it shooting again.

 

[acudaowner]

factory ammo ? you sure things did not loosen up , are you shooting well that day could it have been you or maybe a combo of you weather and ammo ? i ask cause I started thinking my 6.5 cm was comming to the end of the life on this barrel but quickly changed my mind after other more qualified shooters showed me there was not a thing wrong with my gun and 6k + rounds or not it still shot fine . good luck figuring out what you got going on and correcting it .

 

[JMGlasgow]

You have 6000 rounds through a 6.5 barrel?

 

[Baron23]

You have 6000 rounds through a 6.5 barrel?

????

It’s got ~300rds through it.

 

[Marine52]

Fouling and throat erosion degrade accuracy gradually. Not your problem. This is a sudden drop off in accuracy. Looks like you have checked all the obvious things: loose base, rings, etc.
As others have suggested, try another scope. Also, make sure the barrel is not coming loose:

Erratic POI? Check Your Scope — But It Could Be a Loose Barrel « Daily Bulletin

Are you seeing unpredictable changes in Point of Impact on your target? Think you may have a scope issue? Well maybe not -- when was the last time you checked your BARREL? Yes scopes do fail, and scope bases/rings do get loose.

bulletin.accurateshooter.com

 

[JMGlasgow]

????

I was asking @acudaowner

 

[seansmd]

Did I see mention of any muzzle device, brake or suppressor? If so and I missed it apologies but an inspection of those or just removing to test would be interesting.

 

[candyx]

You want to have copper in your barrel just not carbon.

 

[Lost_Phil]

Did you try with factory ammo?

I think it might be your reloads. How many times were the cases loaded and did anneal and trim them at any point?

 

[Krob95]

This wouldn’t be the first NX8 with issues holding zero. Try another scope forsure.

 

[Hobo Hilton]

All good suggestions above. Since we are grasping at straws...

How old are you ? Wearing glasses ? Any noticeable change in your vision (meds) over the past year ? If you let someone else shoot the weapon, how is the pattern ?

Hobo

 

[seansmd]

All good suggestions above. Since we are grasping at straws...

How old are you ? Wearing glasses ? Any noticeable change in your vision (meds) over the past year ? If you let someone else shoot the weapon, how is the pattern ?

Hobo

He said he shot other similar caliber rifles in the same timeframe with expected results. So eliminating some shooter error contributions maybe.

 

[99106]

Change scope, check action screws, barrel to make sure they are not loose. Buddy had similar issue. I asked to shoot his gun, one shot and his ocular fell off. Problem identified. Yours probably not as drastic but I would question the scope if everything is torqued. If it has zero stop make sure screws are tightened also. Since you reload, maybe start with fresh brass. Maybe headspace or neck tension issue?