Slug or just buy the biggest bullet?

Personally, I don't think that Brinnell Hardness 16 bullets are that soft. But, that said, how fast one is trying to push them becomes the deciding factor. You haven't indicated what your load is or how fast you are trying to push them, and these could also be factors for wide variations. Are you getting excessive leading in the barrel?

Slugging the barrel to get an accurate dimensional measurement is the best, scientific method to use. Otherwise, you're just making a wild guess, IMO.

For your Marlin and the micro groove barrel most times it takes a touch bigger diameter for them to engrave properly in the shallow rifling. Also the specific shape will have an impact as well.

There are plenty of folks over at Castboolits shooting the Marlins, and even one fellow who had Lee molds expressly built to suit them. In most of his designs the overall differences were larger diameters, and bullets with the WFN design to better engage and grip the micro groove rifling.

Also as mentioned you might not want the 18bhn verses something around a 12 or 15. IT will depend on what your trying to accomplish and how hard your driving them as well as the lube being used too. All added together will bring the group size down.

...given that even BHN 16 bullets are pretty soft...

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A BHN of 16 is NOT "pretty soft". Remember, the original .44 Magnum used bullets with a BHN of about 11.

Don

41 Mag said:

For your Marlin and the micro groove barrel most times it takes a touch bigger diameter for them to engrave properly in the shallow rifling. Also the specific shape will have an impact as well.

There are plenty of folks over at Castboolits shooting the Marlins, and even one fellow who had Lee molds expressly built to suit them. In most of his designs the overall differences were larger diameters, and bullets with the WFN design to better engage and grip the micro groove rifling.

Also as mentioned you might not want the 18bhn verses something around a 12 or 15. IT will depend on what your trying to accomplish and how hard your driving them as well as the lube being used too. All added together will bring the group size down.

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there's a LOT of guys over there casting with oversize molds, sizing with modified sizing dies, and customizing molds with HUGE meplats..

you should start by slugging your barrel, yes.
But before you go to the expense of molds and sizer modification,
Bear Tooth Bullets will custom size four or five different diameters..

so that might be a problem. Besides that?

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A .359" might bulge the case enough they won't chamber in some guns.

rc

If the gun has a correctly reamed chamber, a .359" bullet will not bulge the case enough to interfere with chambering. In the unlikely event it does, the correct fix is go run a finishing reamer through and not to use undersized bullets.

16 BHN bullets are prolly the hardest you'll ever need for a handgun round. A better BHN is 12. It obturates well at low pressures and is plenty hard for 1500 FPS.

The size of the bullet will depend on the freebore diameter. You can slug that yourself by bumping your bullet up in diameter, loading it into a case to make a dummy round, tapping the dummy round into the chamber, extracting it, and measuring the indentation. Ideally, you want a bullet .0005" under freebore diameter. Such a bullet will always be larger than your groove diameter, but will seal the freebore and eliminate leading.

There are several considerations when it comes to leading: bullet to bore, bullet to freebore, bullet hardness, lube, powder, and pressure. Most people ignore bullet to freebore, but it is just as important as bullet to bore fit.

When loading lead bullets ya gotta know the facts. Slug the barrel, so you'll know what you're dealing with, no guesses! I have 5, .44 Magnums with 3 different groove diameters, from .429" to .433". If I didn't know exactly what each barrel (and chamber throat) measures, I'd have problems. Just shooting the largest doesn't always work. Why not do it right and know fer sure?

BTW, bullet to gun fit is much more important than bullet BHN, IMO...

ote: "Ideally, you want a bullet .0005" under freebore diameter. Such a bullet will always be larger than your groove diameter, but will seal the freebore and eliminate leading.

There are several considerations when it comes to leading: bullet to bore, bullet to freebore, bullet hardness, lube, powder, and pressure. Most people ignore bullet to freebore, but it is just as important as bullet to bore fit."

Well, good advice, but even if the bullet is bigger than nominal bore size (slightly larger than freebore) in fact it may well be sized down (below nominal size) if there is a constriction where the barrel screws into the frame (revolver)! There may also be constrictions under any stamped lettering or under sight locations. The only way to find out, if and how much, constriction exists is to slug the barrel at the muzzle and then push one all the way through, muzzle to cylinder gap and compare the two slugs.

The OP is talking about loads in a 357 Marlin lever action rifle, I believe. But, all of these items we've posted should still apply whether rifle or revolver. It is not unheard of for some Marlin barrels to have a constriction near where the bbl threads into the receiver, IIRC

Quite so!! Thanks for pointing that out.

Slug.

Yep.