Gentlemen-

Need some advise from the experts - I am a hobbiest gunsmith with a task at hand - I need to enlarge the chamber of my .38-55 Marlin by about .003" toward the mouth end of the case.

Background: Tuning for using cast bullets. Have a barrel that measures .3809" in the grooves. A bullet that goes .382". Unfortunately the chamber is cut too small and doesn't allow a cartridge with above bullet seated in the case to chamber with necessary clearance. I made an impact chamber cast, did the math, and have determined that the chamber is .003" too small at the case mouth end.

Question - what to use to open the chamber up that .003"? I'll be using hand tools only since I have no lathe.

I've looked at adjustable dia. reamers, Flex hone tools, and have given thought to making a hone using a cartridge case, leather, and lapping compound. Are any of these methods suitable? Is there a better way? Any specific gotcha's to beware of?

Thanks for your guidance!

Sky C.

What is an impact chamber cast? I can't imagine a chamber that will not allow a bullet of the proper diameter to be chambered. If this is truely the case I would let a gunsmith with the proper tools and a reamer to open the chamber. .003 is a lot of material to remove with a hone, and to try and hand ream it sound like a lot of risk involved if the gun means anything to you. Trying to get just the right amount of material off in just the right place sounds to me like an awsome task. If you bellied the chamber at all, I would think that you would really be in a jam. Good luck and let me know what you do and how it turns out. Be sure and check that this is what honestly has to be done, it sounds kinda funny that that wouldbe the case. Bob

Do you need to LENGTHEN the chamber, or open the THROAT diameter?
One takes a chamber reamer, and the other takes a throating reamer.
Sounds like you need a good Black Powder Cartridge gunsmith that has the proper tools and EXPERANCE to do the job right (you can't put metal back once you remove it)

Hello Gentlemen:

I need to open the chamber area that holds the case neck - not in front of the case mouth, and - not in need of lengthening the chamber.


Marlin designed the rifle to old time specs when BP was the only propellant (large in the barrel grove dia. - small in the chamber). In that case - an under diameter CB was cast of soft lead and the BP gave it a smart kick in the seat causing it to obturate and fit the bore.

While there are folks out there that loves the "holy black" - I for one am happy with the less fuss'n'muss of modern smokeless powders. I want to modify the chamber so that a bullet of larger diameter (.001" over groove dia.) can be loaded into a case, have it chamber, and leave .001" clearance in the neck area.

This problem is fairly well known to those with .38-55 Marlins. Today - CB's are typically sized to be .001" to .002" over groove diameter. Many folks that have tried to load .379" bullets (even though they are still too small) to shoot in the barrels featuring .380"+ groove dia. found out they could not chamber the round. Marlin knows this and will ream out the barrel chamber upon request. I had them do that already but the chamber is still too small for me to use a proper size bullet for the groove dimensions on the rifle.

I am simply wanting to open up the chamber area in the area of the case neck to avoid the brass crimping onto the bullet chen it's chambered.

Here's what I have now:

Chamber dia at case neck: ..3980"
Bullet dia. (.382") + brass thickness x 2 (.018") + minimum neck clearance (.001") = .401"

As you can see - I have an interference fit now - not good for chamber pressure or the health of the gun or myself.

Removing .0015" off the chamber walls would give me the .003" in dia I'm looking for. Does .0015" seem like a lot of material to remove with a hone?

Thanks-

Sky

Yes, .0015/side is a lot with a hone.

Could you reduce the mouth thickness? .018 sounds thick to me (used to "normal bottleneck" cartridges with .012-.014 neck thickness) Turning the brass to .016-.0165 should leave you plenty of wall, and eliminate your neck pinch issue.
This might not be a acceptable answer to you however.

I think you would be there for a while with either a hone or a lap but that might be the best way to accomplish what you want without the use of a lathe. I think I would split the front of a case and use a wooden plug to establish the correct diameter. I'd use a 240 grit lapping compound I think. Drill and tap the back end of the case and you could run it with an electric drill. Probably have to use a couple of plugs to cut about 1/2 thou at a time. Important to stay out of the throat. I'm sure it's already big enough. A viable alternative would be to cast a lead lap, split the front, and wedge it open to establish size. Regards, Bill.

If you have one of the new Marlins with the small chamber neck and large groove diameters, then Marlin might replace the barrel. Contact them. I heard about that one time.

Geo.