I'm sure it's not an original idea but I've made no successful searches for info. I want to make a muffler for an OS FS .70 out of a rifle shell casing. I know the id needs to be just under 11mm or maybe even 11 before the threads are cut. Anyone know a common casing to suggest?

That's getting on for 50 cal. Pretty big casing!

not sure what 11mm/43cal size is available. Perhaps think about silver soldering a reducer

DF

I'm planning to cut off the primer end and leave the reduction for the shell for some back pressure. Basically I think you're considering it backwards DerFly?

The case head diameter for standard rimless center fire rifle cartridges is .473 which is just over 12mm. That should give you an ID pretty close to 11mm.

Examples of standard .473" case head cartridges are:

All of the .308 based cartridges. .308, 7mm-08, .260 Rem, .243 Win.

All of the 7X57 based cartridges. 7X57, 6mm Rem, .257 Roberts.

All of the 30/06 based cartridges. 30/06, .338/06, 35 Whelen, .280 Rem, .270 Win, 25/06.

That should give you lots of options for case volume, length & outlet diameter.

The .308 family will have the straightest bodies W/the least amount of taper. They will also be the shortest.

Drill out the primer to correct size, then thread for the exhaust elbow ??

Is this what I understand what you want to do?

DF

So many ways to skin this cat, including the suggestion above which would however, suggest some type of clamp arrangement onto the exhaust header pipe.

It really depends on what tools are available to you

The method you mention would be the most simple DerFly but I don't think that would leave much material for the threads. I hoped to cut the end cap off and thread the id of the casing. I may have to choose slightly larger and fill with some brass or solder then drill and tap.

If I had to choose right now I would likely go with a 30/30.

I just measured the extractor groove of some '06 brass. The groove is .406", too small for 11mm threads.

If you trim the the case head off just ahead of the groove, that will leave you with about 5/32" of "web" for threads. Not a lot.

If you fill the extractor grove W/brass, then drill out the primer pocket & tap, that would give you a solid 1/4" or more of threads. Probably enough if a jamb nut is employed.

The 30/30 is only .422", 10.72mm ahead of the rim so that won't work.

Here is a CASE DRAWING for the '06. It looks like you could get threads way up into the case wall W/O getting too thin.

if you want a smaller exit diameter go W/ .280, .270 or 25-06.

I would likely go with something like a 458 Winchester magnum case, or a 338 Winchster magnum case or maybe a 378 H&H too (others in these big size ranges too). These have case sizes exceeding .411 inches. A .308 Winchester case itself is a little on the small side though.

Some of the other big game super cartridges would work too. You could get some big bore rifle case blanks and resize them down as needed too.

That's a skinny muffler. And thin walled brass. I would look for something at tobacco shop. Those cigar metal tubes are sturdier, and slightly larger in diameter. Easier to work on, imo.

This one may in fact be a pipe dream. After having a couple of common shells in my hand I realize they aren't much more than straight pipes. A shell the size I would want would have to be military surplus or something. Maybe I'm just being cheap. Kinda wanted something unusual also.

One of the various WSM CASES might be the ticket. They are .5550" @ the base.

I think it is the extractor groove on the cartridge cases that cause the problem. Many cases would work but the extractor groove is just around that .411 size so that there wouldn't be much if any brass left to thread it with.

I think that one of the .300 to .350 size magnum cartridges would be about as small as one could go. The 8mm Remington Magnum looks good with a groove of about .475". The .300 Winchester magnum and .300 Weatherby are some other choices.
The .350 Remington Magnum is another one with a extractor groove of .475" too.

Another thought are some rimmed cartridges such as the 7.62x54R Moisin Nagent cartridges with a .486" base dimension. The neck narrows down to about .30 caliber so it has a smaller opening for the exhaust.

A brass shotgun shell might work too. One could machine a plug for the one end to narrow the exhaust outlet down some.

In post #8 I listed the actual measured extractor groove diameter of GI 30-06 brass as .406. I just measured some more cases.

Federal 7mm-08 case = .406", Speer .243 Win = .406", R-P .280 Rem = .401", Hornady .243 Win = .394".

All definitely less than the 11mm target.

While a .375 H&H based belted magnum case has a head diameter of .532", the actual case base ahead if the belt is only .514". The 375 RCM is a full .532, the RUM & WSM cases are .551" & .555" respectively.

The short WSM case would be the easiest to come by, the longer RUM are a bit rarer. Your 12ga brass case is interesting as it would have an OD of about .800". Where would you find a single 12 ga brass hull?

I got my hands on a 300 WSM. Still too small compared to the factory muffler. I might as well just have an extension on the header for the same effect as far as function. My next idea is a CO2 cartridge.

Awfully heavy since it is it is built to withstand pretty high pressure unsupported by a firing chamber like a shell case.

Yeah I was thinking you might have to go with a big cartridge like a 458 Winchester Magnum or a 460 Weatherby cartridge, in order to get the volume up. But then one could spin the neck down some or make a plug to insert into the neck too.

It may work just fine for you. I think you ought to try the 300 WSM cartridge out and see what happens. I am running a old Enya 1.20R with a straight pipe or tube on the exhaust. It is crimped on the end and has a bunch of little holes in it. The engine doesn't seem to be bothered by it. Four stroke engines are not all that sensitive to the exhaust setup like two strokes are.

.458 Win Mag is a 375 H&H head size. Smaller than the WSM or 375 Rugar.

I also think he should try the WSM.

Just go with a mousse can muffler, or an aluminum beer bottle.