Accuracy problem with a BSA Martini

[Slash5]

Finally broke down and bought a beautiful BSA Martini that's been at the local gun shows. Custom wood, fully reblued, rebarreled and chambered for 357 Maximum.

I've got three BSA Martini .22 rimfires and know how accurate they can be. I thought this would be a good gun to get my feet wet on centerfire - cheap to shoot, reasonable recoil, make a good short range, fun gun.

Minor problem, this thing shoots 3 inch groups at 25 yards. I'm using 357 magnum cartridges and have tried 3 different brands and bullet types.
I'm going to talk to the guy who sold it to me at the next show - a part time gunsmith who builds up guns. But assuming the the usual gun show rules apply - warranteed for 30 feet or 30 seconds - what should my next step be?
Slug the barrel, check the crown, check the chamber, check the headspace, hang it on the wall? I'm out of my league here, need some advise.

[Double D]

How is the forearm attached and bedded?

Single shots forearms should be free floated away from the front of the receiver.

Are you using factory ammo or reloads?  What weight bullets

25 yards?  This isn't a pistol, move the target to 100 yards and tell us how it shoots

Yes check the crown.  What does the bore look like?  It's hard to check the bore/throat in a Martini.

What are you using for sights? Who mounted it/them?

3 inch groups after testing 3 factory loads mean nothing.

What did the primers look like. Were they flat, extruded, backing out.

Anytrouble extracting cases?

Did the guy who sold it to you tell you what loads it shot.  

The 357 Maximum is not consider a safe cartridge to put in Martini Cadet anyway. It is a common conversion by Gunsmiths who have little knowledge of the mechanics of the Cadet. It is to long and requires thinning the breech block feed ramp to get the cartridge in the action.  This reduces the breech block strength and allows the block to flex. (not good for accuracy) It also thins the metal over the striker channel and a pierced primer can blow it out.  I have even seen these with a ripple in the feed tray caused by striker spring riding inside the channel. Gunsmiths also grind metal off the bottom of the block so the block will drop further, but this affects secondary extraction.  

Sorry...

[445supermag]

Well now not bad groups useing 357 mag ammo in a max chamber. Try useing 357 maxium ammo made by remington. Rifle and pistol ammo is not like shot gun ammo. 38 in a 357 chamber is not bad but 357 in a max chamber is way to big a jump and will ruien a chamber. Never was the dan wesson recommendation to shoot 357 mag ammo in the 357 rem max chamber. Brass and ammo can be found and you can use 357 mag dies to load 357 max brass. One you touch off a 357 rem max in a martine cadet you will know power. Factory loads are the same pressure as 44 mag.s and loading manuals keep the same psi.  AA9 and H110 are the powders for it. IF ,n you don,t like it let me know but it should make a nice huten rifle. You could load it with black and callit 38 ex long for them shutzen fans.