I have a centenial (1994) Trapper in 44 Mag. I originally planned to use it Boar hunting, but I found that I was more accurate at 50 yds with my S&W 629 Classic (5" barrel), so the Trapper just sat until recently. A friend told me that those Winchester Trappers had Micro-groove barrels and usually did well with .431 bullets. I tried "slugging" the barrel and found that indeed it has a micro-groove barrel. The sizing came out to .430, but I don't trust that as the lead slug was tight at the muzzle then virtually slipped down the balance of the barrel, so I may have an unusual barrel problem. I thought I'd go ahead and try building some .431 based rounds, but ran across this site first and thought I'd see if anyone else has run into a similar problem, and or has experience with the 16" micro-groove barrel Trappers.

Micro-groove barrels on a Winchester?

Micro-groove barrels on a Winchester?
Yep! 12 lands and 12 grooves, extremely shallow. I have no idea whether this (and other Trapper 44 Mag) barrels were procured from Marlin, or made specifically for Winchester. Likewise, I don't have any idea how many Trappers were outfitted with micro-groove barrels. I do know that mine is. I purchased the rifle new in 1994, so there were no after market changes. I don't really care; I just want to make my rifle work well.

Your findings are pretty much the norm for those little carbines! In fact, you'll find that they shoot a WHOLE lot better with cast bullets when fed a steady diet of .432" diameter projectiles! The throats, while short in length are huge in diameter, and that coupled with the generous groove diameter of the shallow groove rifled bore dictate big bullets. Give some .432" bullets a try and you'll never look for anything else!

God bless,

Thanks! I'll do that and report back the results.