It looks like I'm going Mad Pig on my GBL.

Their standard procedure is to cut at 14.5" and pin a brake.

My thought was to shorten only as much as keeping the 6 round tube will allow, and thread to 5/8 - 24, and time a brake.

Is it worth it to lose 1 round in the tube and shorten to approximately 16"?

Is it worth it to lose 2 round capacity and shorten below 16" (pinned brake to be added)?

I looked into threading my SBL. Not sure how yours is attached, but because of how the mag tube is attached in a dovetail slot on the SBL it didn't seem feasible to have enough room to just thread and not trim the tube and barrel. I know some have managed to do it, but I didn't like how it looked being all jammed up at the end. I think trimming the barrel back behind the dovetail would be the way to go. No way I'd trim to 14.5" though and I certainly wouldn't want to be stuck with a brake pinned on. I couldn't care less about losing a round or two of capacity and I thought about going to 16.5" but decided I wanted to keep it unmolested.

Bump

You will kill resale, who wants a brake on a 45-70 cannon with reduced capacity?  

It's 18inches now?   a few hundred $ to lose 1-2 rounds, one pound, and add a brake most people would never want?  Doesn't sound like a good investment to me.  

I'd leave it unless you need to thread for a silencer.  If you just want to do it then why are you asking strangers for their opinion?

Because their guns are awesome, a brake makes recoil more manageable for follow-up shots, and 1-2" off of a 150 yard gun isn't affecting intended range.  

Sounds like you've made up your mind already.  

16 inch with the brake not welded on is far more useful and resellable, that is what I would choose.  

Their website says they charge around $1800 to install parts, paint the gun, and smooth the action over.  You asked for opinions, and I think you're nuts, rich, or both to spend $2500 or more on a marlin.  I would sell your marlin and buy a big horn 500s&w if you want a "thumper" - same price, nicer rifle, plus bragging rights to having a 50cal.  

Matter of opinion, I guess. Their guns look like tacticool crap to me. Not sure a brake is of any use on a lever gun. You still have to work the lever for a follow up shot. If mounting a suppressor I'd just go DT. But, hey, you seem to know what you want so go for it.

This about covers it.  16" threaded is useful down the road, might as well get maximum versatility from your expensive rifle.  Their thumper setup seems neat, but also makes my shoulder hurt just thinking about it.  A 14.5" P&W on a .45-70 seems like a stupid way to limit the versatility of the cartridge, thoughwhat if you want to change suppressors or muzzle devices?

The big horn .500 idea also seems neat.

Reload slower rounds.

1,600 fps vs 1,400fps on a RNFP won’t matter.

A 405gr lead bullet dropped buffalo at black powder velocities (1,350fps) in 1880.

Buffalo are pretty damn big.

If it’s bravado, then why the brake? If you need faster follow up shots then lower the charge.