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What loads would be recommended for an 1927 12 ga drilling

Whichever loads you choose, it would pay to have the bore/chamber checked as I believe civilian guns could be chambered in either J(.318) or JS(.323) bore.

GDU

If it's a 8x57JR, you can get factory ammo from Sellier and Bellot for only 11 bucks a box!

The has 7.8 mm =.30709 stamped on the barrel

In my experience, the 7.8 mm notation on the barrel stamps of a prewar german or austrian rifle denote the 8X57 J caliber. if its a rimmed round (almost always for a drilling or combo gun) then is the .318 diameter 8X57JR cartridge.

The good news is you can fire a 8X57JR in the larger rifle with no worries. BUT NOT THE REVERSE. course you know that.

FOr the price, I would buy some Sellier bellot rounds, shoot into a sand bank or other recoverable medium, and then mike the bullet base. I believe you will find the grooves mike 7.8 mm and the lands mike 8mm at .318. if not stop and go slowly.

You can have a lot of fun developing both bullets and loads for the .318 JR round, if you purchase a bullet swage device. With this, if you lube the bullets, you can compress a .321 down to a .318 or .319 or whatever you determine to be best for your rifle. Not with solids of course, but with the typical run of 8mm bullets I have had good success.

You need to determine for yourself, but I find that dropping 10% off of the Minimum 8X57 Mauser loads, and loading up towards the low end of range, is not a bad range.

Best bet is to buy the S and B. Take along a chrony, and then load to that velocity with the same bullet weight. 196 grains is the standard for your time period, adn this is probably denoted on your flats by 14.4 grams, or something similar, which is teh metric equivalent for bullet weight. Only the marks on your rifle can tell you for sure.

Good luck. send me an email and I can share some loads I use with my Merkel prewar rifles for the same caliber. I dont do loads over the boards.

7,8 mm means land bore diameter plug and in real means 8x57J or JR, (.318")
7,9 mm means 8x57JS or JRS (.323"), but before the war it was millitary standart and allmost never been used for hunting purposes (sometimes for sporting).

Thanks for the info. it is an 12-12 8x57Jr,would the gun handle modern shotgun loads?

My guess would be that the shotgun barrels are chambered for 65mm (2-1/2 inch) shells. You need to have the chambers measured to determine their length. Don't even consider opening the chambers to 2-3/4". Although you probably won't weaken the barrel significantly, why take the chance?
2-1/2 inch shells are readily available as loaded ammo or components. I use Fiocchi 16 gauge cases to load for my drillings using a slightly modified MEC 600 Jr.

8mm=.315, which is neither "Actual" bore or groove diameter of either the 8mmJ or JS, but simply a "Nominal" number. I seem to recall seeing the German S bore was originally called the 7.92x57JS & not 8mm. The marked dia by the proof house is the Bore dia & as Geno so rightly pointed out 7.8 almost certainly designates a J bore while 7.9 would be an S bore. I have an 8x58 Sauer which was marked prior to 1912 under the gauge system & is marked 172.28 which translates to .300". One realizes at this point in time bores were simply marked by the largest plug dia they would accept & the next one up was 156.14 or .310". This therefore does not mean the gun had a .30 cal bbl, but is indeed a .318" 8mm having a proper bore size of around .307/8.