Bullets - .458"

Now we are getting technical. You need to slug your barrel, or make a Sulfur cast of your chamber. In short, slugging your barrel is smashing an oversized lead ball or bullet through the barrel and measuring the largest diameter on the resulting slug. Add .001 or .002 inches and get that size of bullet.

Google "Slugging your barrel" or "Make a sulfur cast" for detailed instructions.
 
I've been searching the internet for more information, and some people have slugged their rifles and decided for a large .460 cast bullet for their marlin 1985G
Let's say my rifle also measures that high.
So why a jacketed or a berry plated bullet in .458 would work well with it?
I'm not doubting it, just trying to understand...

Thanks
 
If it's a modern Marlin with Micro Groove rifling then you have a whole other can of worms on top of the normal stuff.

The short version of things to deal with Micro Groove barrels is to use the hardest bullet you can find, make sure it's at least .002" over rated bore diameter if not .003" or even .004", and keep velocities under around 1600fps even with a gas check. Because MG rifling is so much shallower than regular rifling there is less grip from it and the bullet can skid through the barrel instead of engraving properly if it isn't hard enough and/or big enough.

I don't have a MG 45-70 but I do have 44 mag and 30-30 Marlins with MG rifling. In the 44 mag rifle I tried .429", .430", and .431" and they all leaded badly and had horrible accuracy at any velocity from 900-1900fps. I picked up a mould meant for a 44-40 that drops 200gr bullets at .434" and they shoot great up to around 1500fps (not a gas check design). My 30-30 with MG rifling shoots .308" jacketed just fine but for cast I use .311" and have 1" groups at 50yd with no leading. Even with the .311" bullets I have to keep it under 1500-1600fps to maintain any sort of accuracy.

A modern 45-70 is supposed to be .457" so if going with cast bullets with MG rifling I wouldn't look at anything under .459". You can go with a lighter bullet like a 300gr but will have to keep velocities down. I shoot jacketed 300gr bullets out of my 45-70 Handi Rifle at 2350fps within safe lever pressure levels so it is possible to get them really trucking along. At ~1600fps and below you really don't need a gas check but make sure the alloy is a hard one; anything listed as "hard cast" should work. You could slug the bore but I find it really isn't necessary with modern, commercial rifles. I just make sure that cast bullets are .001"-.002" over normal bullet diameter for regular rifling and .003"-.004" for Micro Groove rifled barrels.

The copper jacket on a normal bullet is much harder than lead, even hard lead, so it can engrave better against the steel of the barrel. Lead, if too soft or moving too fast, can skid and scrape along the barrel instead of engraving and turning with the rifling. Berry's plated bullets have a pretty thick copper plating on them that makes the outer layer harder and less likely to skid.
 
Thank you for all the info.
It makes sense now.
My Marlin is fairly new, Remington built and the barrel has deep-cut Ballard-type rifling (6 grooves)
I will slug the bore soon to find out the size.

LUTNIT said:
If it's a modern Marlin with Micro Groove rifling then you have a whole other can of worms on top of the normal stuff.

The short version of things to deal with Micro Groove barrels is to use the hardest bullet you can find, make sure it's at least .002" over rated bore diameter if not .003" or even .004", and keep velocities under around 1600fps even with a gas check. Because MG rifling is so much shallower than regular rifling there is less grip from it and the bullet can skid through the barrel instead of engraving properly if it isn't hard enough and/or big enough.

I don't have a MG 45-70 but I do have 44 mag and 30-30 Marlins with MG rifling. In the 44 mag rifle I tried .429", .430", and .431" and they all leaded badly and had horrible accuracy at any velocity from 900-1900fps. I picked up a mould meant for a 44-40 that drops 200gr bullets at .434" and they shoot great up to around 1500fps (not a gas check design). My 30-30 with MG rifling shoots .308" jacketed just fine but for cast I use .311" and have 1" groups at 50yd with no leading. Even with the .311" bullets I have to keep it under 1500-1600fps to maintain any sort of accuracy.

A modern 45-70 is supposed to be .457" so if going with cast bullets with MG rifling I wouldn't look at anything under .459". You can go with a lighter bullet like a 300gr but will have to keep velocities down. I shoot jacketed 300gr bullets out of my 45-70 Handi Rifle at 2350fps within safe lever pressure levels so it is possible to get them really trucking along. At ~1600fps and below you really don't need a gas check but make sure the alloy is a hard one; anything listed as "hard cast" should work. You could slug the bore but I find it really isn't necessary with modern, commercial rifles. I just make sure that cast bullets are .001"-.002" over normal bullet diameter for regular rifling and .003"-.004" for Micro Groove rifled barrels.

The copper jacket on a normal bullet is much harder than lead, even hard lead, so it can engrave better against the steel of the barrel. Lead, if too soft or moving too fast, can skid and scrape along the barrel instead of engraving and turning with the rifling. Berry's plated bullets have a pretty thick copper plating on them that makes the outer layer harder and less likely to skid.
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