Lousy groups

[bob4wd]

Have a couple of Mosin snipers, one will consistently deliver 2inch groups at 100 yards but the other one ———really, really bad. Typical groups are 8 or 9 inches!
The really irritating thing is there will always be 2 or 3 shots (out of 10), that group an inch or two in every bunch. The bore is excellent, no pitting at all. Action is tight. Tried with and without hand guard, no difference. It has had metal shims installed at some point in it’s history, usually a good sign. Tried letting the muzzle end free float, tried shimming it. No difference.
It’s an older Century import Ishevsk, 1943, modern Russian scope, Mount is matched to the action.
Any ideas? Recrown the muzzle? Back-bore for an inch or two (would hate to do this)?

Information

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[Gil Boyd]

Bob,
If I was with a student, I would put those rounds down to incorrect holding of the weapon, changing POA or simple movement each time, which we all do.
Don't do anything to the rifle until you have tried it on a bench for a good sound base.
Let us know how you get on, it should tighten them up nicely.

[Ovidio]

Did you try it with its iron sights? Does it have the same spread?

[mr.e moose]

Check the screw on your mount. I shot 4th at a service rifle shoot only to discover after that the screw had loosened.

[bob4wd]

Check the screw on your mount. I shot 4th at a service rifle shoot only to discover after that the screw had loosened.

Scope mount and action screws are all tight. All shots from a bench on sand bags. Haven’t tried the open sights yet, that’ll be next. By the way, results are pretty much the same with light loads (lead), fairly heavy loads (lead), factory ammo and reloaded ball. Wrapping tape around the muzzle-end seemed to help, but then reverted back to the usual shotgun pattern!

[Gil Boyd]

Bob,
Sorry to be a pain. Those rounds on the target you showed us are not that bad. Its a shame you haven't got a round by round numbered sequence which could help the diagnosis.
It is a combination of very slight errors, sandbags will help, but I can't see how you, for example squeeeze or pull your trigger snatch your bolt etc!
Just try it one more time with someone spotting for you and number the rounds for me, we'll get there mate.

[Sunray]

What specific ammo? Bullet diameter is important too. Did you work up the load with the milsurp ball? Lotta that is really low end stuff though. Not made for accuracy.
Those shims were put there for a reason. However, it'd only apply to the scope and mounts used at that time. Not whatever scope and mounts Century cobbled together.
Mind you, I'd be suspicious of any rifle Century had anything to do with.

[Patrick Chadwick]

As you can shoot well enough with a different Mosin, we can assume that you are not the problem. And if the rifle is OK at the front end, the trouble could be in the back end. The rifle may be badly worn in the throat, in which case you may need to go to hand-loaded ammo, as most factory ammo uses boat-tails.

Find out the maximum possible cartridge length - the method has been described several times in these forums, so search it out. If the max. length, i.e. the point where the bullet is just about touching the lands, is so far out that the bullet is completely free of the neck of the cartridge case, then the rifle will never shoot well with that bullet. Basically, the bullet has a substantial free flight in the worn throat, hits the transition cone at an angle, is engraved crookedly, and comes out of the muzzle with a skew that means the POI is all over the place! Hence the extraordinarily large group, which nevertheless has the centre in th emiddle of the target.

For a very worn throat, about the only pracatical answer is to use a flat-based bullet with the largest diameter that can be safely used in the chamber and the longest possible cylindrical section. The best choice is the Hornady #3130 flat-base round-nose 174gn bullet with a diameter of 0.312". They seem to be rare these days, but I think there is one from Speer that is similar.

[Patrick Chadwick]

Re. maximum cartridge length. I looked it up myself. Here is the thread

https://www.milsurps.com/showthread....ight=boat+tail

The detailed explanation of what is (possibly) your problem is in post #139.

[bob4wd]

As you can shoot well enough with a different Mosin, we can assume that you are not the problem. And if the rifle is OK at the front end, the trouble could be in the back end. The rifle may be badly worn in the throat, in which case you may need to go to hand-loaded ammo, as most factory ammo uses boat-tails.

Find out the maximum possible cartridge length - the method has been described several times in these forums, so search it out. If the max. length, i.e. the point where the bullet is just about touching the lands, is so far out that the bullet is completely free of the neck of the cartridge case, then the rifle will never shoot well with that bullet. Basically, the bullet has a substantial free flight in the worn throat, hits the transition cone at an angle, is engraved crookedly, and comes out of the muzzle with a skew that means the POI is all over the place! Hence the extraordinarily large group, which nevertheless has the centre in th emiddle of the target.

For a very worn throat, about the only pracatical answer is to use a flat-based bullet with the largest diameter that can be safely used in the chamber and the longest possible cylindrical section. The best choice is the Hornady #3130 flat-base round-nose 174gn bullet with a diameter of 0.312". They seem to be rare these days, but I think there is one from Speer that is similar.

A very good thought indeed. All of the bullets I used are flat base, and are loaded on the long side but not fitted to this rifle specifically. Will go though the process and check ‘em out!
As for bullet types the lead loads were of three diameters; .312, .313, .314, all 180 grains and gas checked.owder was 2400, 14,and 18 gns. The ball was pulled Russian 150 gns. Flat base. Didn’t measure diameter. Charge from Lyman manual, min. Load of H4895, I forget the weight and am away from the bench right now.
It’ll be a couple of weeks before I get to try anything, on my to Phoenix for the big gun show coming up this weekend. Will report back.
I don’t really know why I’m trying to make this gun shoot when I’ve got a perfectly good one ( not a Century POS, by the way, but a fine semi-real one to shoot), but it’s a challenge that is becoming an obsession!