CZ 457 MTR problem

[recumbent]

A friend has a CZ 457 MTR 22LR. It shoots great using Eley ammo it will shoot 1 hole 5 shot groups for 15 shots at 50 yards, then the groups open up
to the size of a silver dollar. Action is torqued properly ect we cant figure out what's causing this.
Any suggestions.

[NSB]

The barrel needs a very, very good cleaning. It’s leaded up.

[tazman]

I think NSB may be right. A good cleaning is in order.
I am not certain if it is leading or carbon fouling. A good cleaning should take care of both issues.
The chamber on the MTR rifles is very tight. Any buildup of fouling in the chamber throat area will effect accuracy.
The MTR barrels I have seen are smoother than most. Leading, while unusual with ELEY ammo, could be happening. If leading is present, it will show up on your patches as it comes loose during cleaning.

I would also suggest investing in a borescope. They don't cost all that much(about $55-60). They can let you see exactly what is going on inside the barrel.

[recumbent]

He has cleaned it every 25 rounds and has scoped the barrel, barrel looks fantastic inside. Groups go to hell after 15 shots so it gets cleaned after 25 shots.

[NSB]

It’s collecting lead in the barrel from something. I’d suggest running a borescope down the barrel before and after and seeing where it’s collecting. Based on the limited info available it’s not the scope or mounts if it’s happening after 15 good rounds down the barrel. May have a defective spot in the barrel where it’s stripping lead off.

[georgerkahn]

It’s collecting lead in the barrel from something. I’d suggest running a borescope down the barrel before and after and seeing where it’s collecting. Based on the limited info available it’s not the scope or mounts if it’s happening after 15 good rounds down the barrel. May have a defective spot in the barrel where it’s stripping lead off.

I was to offer a similar thought as presented by NSB! I have taken the cotton off a Q-tip and slipped it into the slot of a pull-thru (like nylon fishing line with a loop at end) air-rifle bore cleaner with it -- upon insertion -- to be a snug (no more than "snug") fit. Pulling the line it has been quite easy for me to "find" constrictions as well as loose spots. NOW -- most recently (thanks to tazman suggestion) I have a borescope... However, pulling the Q-tip cotton ball through has worked remarkably well for me on .22s -- including my pair of CZ457s.

[tazman]

As NSB suggested, find out where the problem spot/s in the barrel are located. A borescope will help by giving you a visual of the area.
It may also let you know if there are any rough spots and how bad they are.
My recent experiences with J B Bore Brite/Paste suggest that roughness that harms accuracy can be dealt with by hand lapping the barrel.
I had a CZ 455 Varmint rifle go from about .75 inch groups to .5 inch groups from just a single session of bore lapping.