Did any of you Marlin 45/70 or 450 Marlin owners Firelap or otherwise break in your barrels before shooting cast bullets? What size bullets do you use .458, .459 or .460? Did you slug your barrel first to determine the correct bullet diameter? You out there Griz, mattpair or Stanger73? Thanks.

My first 450 Marlin had a 1,000 to 1,500 rounds of jacketed through it before I tried cast. The bore was butter smooth and I never got any leading. All of the cast I shot in this barrel was commercial 0.459 dia. Accuracy was outstanding.

When I had the gun rebarreled to the non-ported version I slugged the bore and examined it per Marshall's suggestions. It was very rough and had 0.0015" of constrictions under the dovetails and stamped lettering. Shooting cast would have been an effort in futility so I got a lapping kit. I ended up shooting 45 lapping rounds to get where I needed to be. The bore now slugs at 0.4585 with a smooth taper towards the muzzle.

I know cast my own bullets and size them at 0.4605". I've tried them at 0.459, 0.460, and 0.4610". My gun shows a marked preference for 0.4605" and bigger. Bullet alloy, hardness, and lube also play an active roll and alll of this depends on the velocity desired.

I wold suggest that you get a copy of Marshall's book. Slug your barrel with hollow fishing sinkers (barrel type). Do one slug the full legth of the bore, and one just flush with the muzzle then driven out. Measure with a good 0.0001 resolution micrometer, not a caliper. If the muzzle slug is bigger than the full pass one, you got a barrel that needs to be lapped. Only other choice it to shoot a couple thousand jacketed through it first. Personally I would just order the complete laping kit, with book, if you plan to shoot cast. Marshall will want to know what your prelapped groove diameter is when ordering so go ahead and slug it.

BABore: I have Marshall's technical guide, I just wanted to hear what others had to say before going the whole 9 yards. Thanks for the reply. This may sound like a dumb question but are the bullets used for firelapping sized specifically for the measurements you get when you slugged your barrel? I also plan on having the barrel on my Guide Gun cut down and front sight reinstalled to 16" for a 450M "trapper". Would it be correct to assume that I should wait until the barrel is work is completed before firelapping?