"Not to hijack your thread, but I have a 16" Trapper and would like
to know what factory .357 Mag ammo is best suited to the barrel.
I have Gold Dot 125 gr. GDHP and Black Talon 180 gr. SXT.
I have tested the 16 inch Trapper and the 20 inch carbine with ammo from Georgia Arms (professionally reloaded). They used 125 and 158 grain GDHP 357's. With that ammo, the Trapper definitely groups tighter with the 125 grain ammo, particularly at 50 and 100 yards. In fact, the 158's scatter the shots (large group size) even at 25 yards and become laughable at 50 yards. The 20 inch carbine is decently grouped at 50 and 100 yards with the 125 grain GDHP. Not as tight as the Trapper, but still better with the 125 than the very poorly grouped 158 grain GDHP.
Unfortunately, the best groups obtainable with a scope and using a rifle rest at 100 yards with the 20 inch carbine and using Remington 125 grain SJHP is a few inches.
A few points to consider:
1. I don't have a chrono, so I don't have velocity data for any of that.
2. These are lever rifles with mag tubes and barrel bands. They are meant to be hunting rifles (with minute-of-deer sized groups), not marksmanship-demonstration instruments. They are prone to vertical stringing and uneven heating/cooling with varying rates of fire.
3. Interestingly, similar findings were observed with 125 grain 38 Special ammo vs. 158 grain 38 Special ammo at 25 and 50 yards.
I concluded that:
The 158 grain ammo groups lousy with these rifles and the 125 is noticeably better, but not perfect.
Never tried heavy stuff like 180 grain ammo.