Shot my "new" Artillery Luger this morning - and a problem/question

[spacecoast]

Last week I bought the Artillery upper (receiver and barrel) from Don Voigt. Don shipped quickly and I was delighted to receive the package yesterday. I supplied the toggle assembly and frame from my mismatched 1918 Erfurt, added a little extra lubrication and was pleased that everything fit together fine.

I took the gun out today for our club's 25 yard "fun" match - 10 shots each of slow fire, timed and rapid fire. Not surprisingly, the gun shot about a foot high with the artillery rear sight all the way "down" at 100 yards, but here's where the problem came in - as I progressed through the magazine, the shots got higher and higher, and I discovered that under recoil the sight was slipping to higher and higher yardage settings.

Is it possible to get a stiffer spring for the rear sight adjustment - to prevent slippage? Or what would be another possible remedy? Would this situation improve or get worse with an artillery stock? I was shooting one handed, so the muzzle flip was fairly pronounced.

By the way, the gun cycled the 30 rounds perfectly with three different magazines (6 - 5 round strings) using two different reloads (4.1 grains of AA#2 and 5.3 grains of Unique, both under a 115 gr. plated round nose bullet). I'm very happy with the setup (assuming the sight issue can be fixed) and have a reproduction stock/holster on order.

Any suggestions appreciated.