doublegunshop.com - home Forums General Discussion DoubleGun BBS @ doublegunshop.com question for those familiar with 24 gauge guns

I only have a dial caliper but it measures my two 20 gauge A-Zoom snap caps at .667 and ..666 and they will not chamber in my 24 gauge.

Many European guns have tight bores for their gauge, is there any proof mark indicating the chamber gauge?

If it is a 24 gauge welcome to the club of oddballs - yes, I am definitely an oddball. But the gun doesn't care because it's broken clays and killed quail and woodcock for me because I definitely don't deserve any of the credit.

I Agree with all the Old Colonel said on this except for one thing which I will take exception to. That is that most guns which were intentionally made for use with all brass shells were bored oversize not undersized. At the time these guns were made all shells were using card & felt or fiber wadding. The brass shells have thinner walls requiring larger wads, thus the oversize bores. Commonly a 12ga brass shell was loaded with 11 gauge wads, a 16 with 14ga & a 20 with 18ga etc. Note also that ion the very early days wads were essentially bore size so in those days brass shells used 2 sizes over as 10 ga wads in a 12 while a paper shell used 1 size over as an 11.
At some point the makers began making the wads larger than actual bore size so for a 12 ga paper case you used 12 ga wads, for the brass 11 etc.

Admitting you cannot measure the small end of the chamber down at the forcing cone with a dial caliper you can certainly measure it just ahead of the rim seat. This should be ample to determine the gauge. Remember we are looking at a difference of 036" between nominal 20 ga & 24 gauge. It doesn't have to be read to within 0001" to determine whether its a 20 or 24. That could be determined with an inside spring joint caliper & a 6" machinist scale.

By Top Of Bore, I meant diameter B in the referenced chart. Again, the whole thing that changed my thinking that it was a 24 vs a tight 20 was the 15MM proof mark, which is much closer to a 24 than a 20. I am using a RCBS dial caliper to measure, which obviously isn't the most accurate as I initially cam up with a measurement that was .001 under that of the snap cap, which isn't possible!

Per 2-Piper, you are correct I misspoke, traditionally if different sized the bores would be undersized not oversized

If the bore proof mark is 15mm (0.591 inches) That number would make sense as the original choke marking for a 20 gauge which would be .025 from a classic 20ga bore of .615 and makes sense.

The fact the gun near chambers the 20ga snap but not quite, when my 24 won't admit into the chamber any part of a 20ga shell or snap cap. Makes me think it can't be a 24ga.

Really need pictures of all marks and barrel stamps

20 gauge in chamber snap caps in chamber

Hopefully my attempt at uploading these photos work!

That looks like a 20 ga gun with either extra tight chambers or rim cuts which are not deep enough. Since 20 ga empties are easy to find I would try several to see if any close. Perhaps you have just hit on two combinations of shell and snap caps which don't work. I suspect you'll find nothing closes and you will need to have the chambers recut. It is a minor issue to the right gunsmith. Had this been a 24 the odds of anybody having a chamber reamer or rim cutter would be low.

Okay, quick update, I just fished out some old unfired 20 Gauge Remington Kleenbore shells from the sixties and they do chamber...but are tight (I need to pop them out. So i obviously have a 20 and not a 24. I'll send the barrels off to Michael Orlen this week to have the chambers sorted out (without lengthening them). Sorry for the false alarm and hopefully Old Colonel has not yet mailed me 2 24 gauge shells. Many thanks to all.

True, you can get tight chambered guns. I have a John Fry of Derby double 16 bore that needed slight skim out as factory load ammo was a push fit and the black powder re-loads that I need to make for the gun were very difficult to get in. Simple job to sort out. Lagopus.....

Before altering the chamber it might be easier to resize down a batch of 20ga shells to fit the chambers. Plastic shells are not hard to down size.

You can set up the barrels with a wood mallet or piece of 2x4 cut down gently (very gently) tap an empty 20 gauge hull (slightly lube the metal base with either cartridge lube or pledge) into the tighter of the two chambers. Then use a wood cleaning to gently tap it out of the chamber.

Note I believe it is safer to assume it is a 2 1/2 in chamber and not 2 3/4. It is easy to trim a 2 3/4 hull to 2 1/2 then load it and roll crimp it.

Once sized load the with a good 7/8 oz low pressure sub 8,000 PSI load. You can use 2 3/4 inch data, most of which will fit in a 2 1/2 roll crimped hull.

Based on the "65" mark on the barrels it has 2 1/2" chambers if they haven't been altered.