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Discussion Starter · #1 ·
Hello;

Rifle is a Marlin 336CS, 1992 JM version in 35 Remington. Having sudden loss of accuracy. Going from 1.5” groups to 10” after 12 shots.

Bullets used are the Hornady 158xtp (35780), Speer 180 grain fp, and Hornady 200 grain rn. Powders used are H335 & IMR 3031.

If you look at the rifling, from muzzle, you can see machine marks. The Hornady bullet is a .357 diameter bullet, could this slightly smaller diameter bullet the cause?

Cleaning after shooting, barrel has lots residue and what looks like copper.

Thank you any input.

rp
 

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There are many reasons for a sudden loss of accuracy. Was the inaccurate load only with the 158-grain bullet? Exactly what changed between 1.5” and 10” groups (at what range?). What sighting equipment are you using, scope, irons, red dot? “Machine marks”, what does that mean - with or without magnification? Parallel or across the lands? Where the loads which gave 1.5” groups identical for the 10” groups? Exactly how are you cleaning?



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Good cleaning to see if that helps. Any damage to the barrel crown? You shoot any cast lead bullets recently ? Using a scope or iron sights? Scope can go bad and cause this. Are your shots grouping consistent, even though not where you are aiming. Or, are they all over the place with no consistency? Start with the simplest issue and work your way back. Keep us posted.
 

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Discussion Starter · #5 ·
thx for input.

started with leupold 3x9 scope then a nikon 3x9 scope. all loads gave 1.5" group at 100 yards before loss of accuracy. using bags on front and rear of rifle. machine marks are across the lands, no change in reloads when groups went to 10". bullets are flying over the target with no consistency.

bring spares of almost everything at range. have a small dolly to carry everything. spare scopes, dial calibers, all kind of tool. habit from bench rest. 3 shots with each bullet weight, bullets everywhere, switched scopes 3 shots with each bullet weight same thing. stopped shooting, hording jacketed bullets.

following directions on butches bore shine to clean barrel. can see what looks like copper on lands before cleaning. not showing any green on cleaning patches, to black with residue. no sign of copper after cleaning this morning. will not shoot again until next week. crown looks ok. have not shot cast bullets, yet.
i
thx again for input.

rp
 

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thx for input.

started with leupold 3x9 scope then a nikon 3x9 scope. all loads gave 1.5" group at 100 yards before loss of accuracy. using bags on front and rear of rifle. machine marks are across the lands, no change in reloads when groups went to 10". bullets are flying over the target with no consistency.

bring spares of almost everything at range. have a small dolly to carry everything. spare scopes, dial calibers, all kind of tool. habit from bench rest. 3 shots with each bullet weight, bullets everywhere, switched scopes 3 shots with each bullet weight same thing. stopped shooting, hording jacketed bullets.

following directions on butches bore shine to clean barrel. can see what looks like copper on lands before cleaning. not showing any green on cleaning patches, to black with residue. no sign of copper after cleaning this morning. will not shoot again until next week. crown looks ok. have not shot cast bullets, yet.
i
thx again for input.

rp
Sounds to me like a scope mounting problem or an internal scope failure. The machining marks didn‘t show up while you were shooting, and a dirty barrel usually brings a gradual change in accuracy. Since the Nikon scope was put on last, my guess would be something loosened up on the Nikon scope mounting hardware or the Nikon scope itself failed.
 

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It never hurts to really clean the bore. I use JB Bore Paste or even buffing compound on patches. Just make sure to remove it with clean patches and alcohol. You can then oil the bore, leave it dry, or, as I do, run a patch or two of 1000+ down the bore.
 
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