doublegunshop.com - home Forums General Discussion DoubleGun BBS @ doublegunshop.com Loads for combination Guns - 2nd quick shot

I have now worked up a load for the 7x65R Heinrich Munch combination gun. 160gn Speer soft point over 52g of IMR4381, with an overall length of 84mm which keeps well under 1 inch, but only if I shoot slowly and barrel is cold between shots.

Having a thin barrel it does heat up and cool down very quickly, and if I take a 2nd shot without waiting for the barrel to cool it is going three / four inches high. I fully expect that shooting quickly bullets will group quite widely, but just wondering if other loads may well group a bit tighter from warm barrel. Can anybody cast any light on this?

Would dropping down to a lighter bullet weight - say 140 gn help?

Fully appreciate it for what it is, and that in 95% of times one shot is enough, but when shooting female deer you often have to take the following calf at the same time, thus requiring two quick shots.

Changing bullet weight won't likely help. You inadvertantly answered your own question. To wit:

You need two quick, accurate shots. You don't have them. Thus, pass on the doe and the calf if you are carrying the combination gun, since, you only have one, quick, accurate shot.

It is what it is.
Best,
Ted

I've been looking at a drilling by Ludwig Borovnik recently with a full length tube insert, which would be surrounded by air, with 3 set screw adjustments about 1/3 of the way from the business end. So I've pondered this problem for a bit and really haven't applied pencil or physics to it but it seens that the glass is the problem. Basically the rib and the other tube is a heat sink for the lower rifled tube and possibly the whole lot moves. So if you only use iron sights at an appropriate distance would you see a similar variation or a closer point spread with the glass being the culprit? Or split your error with the 1st shot going an inch or two low and the follow up going an inch or two high with repect to the point of aim of the glass.

Kind Regards,

Raimey
rse

I managed to reduce this somewhat with a 7X57R by somewhat reducing the loads using a faster-burning powder. On this gun rifle barrel is soldered full-length to shotgun barrel. Normal load with 175 grain bullet and full load of IMR4350 gave very good groups at 100-200 meters, so long as 5,0 or so minutes between shots. Two really fast shots were OK too, just hold a little lower for second. Almost same results with Norma 150 grain factory ammo. Certainly no problem with big targets in forest, like moose or kronhjört in opening.

However, load with enough IMR4895 and 145 grain bullet to give good velocity (never chronoed) gave less spread between 1st and 2nd shots, enough that with open sights there was no effective change of POI for two, sometimes three shots. Basic reason seems to be that lighter load heated slim rifle barrel less. Still plenty of power for deer at 100-150 meters.

Niklas

Thanks Gentlemen, and in particular Niklas - you have confirmed my thoughts. I have also spoken to one of the manufacturers of Comobination guns and they usually expect 2nd shot to go 10 to 12 cm high from a warm barrel if barrel is soldered to the other barrels. As the barrel heats up it lengthens slightly causing to warp upwards.

Make sense that lighter load with less powder / energy will cause barrel to heat up less.

And what is a kronhjört? - A wild boar?