For what it's worth
In case anyone still sees this, I googled upon this while looking at small game loads for my 308 - ironically I first tested the idea using the very adapter you are asking about.
I ordered 2 of them thinking I at least had the opportunity to reload (or lose) one of them. Neither fit in my normal 308 (savage 110) but both fit in my modified H&R single shot. I did try and make one fit by chucking it at grinding it down a little getting it to fit better and better until either a: I needed to attend to the rest of the gear I was loading for the hunt b: my wife asked me to do something or c: my drink was empty.....
So sticking with the H&R "truck gun" I was delighted to find it shot to the same point of impact w/ the adapter as the iron sights (set for 180 gr core-lokt) out to about 50 yds (minute of mik jug). I already had a .32 so ammo was no big deal and it road around the woods on logging roads on the Olympic Peninsula. I shot both Ruffed and Spruce grouse with it and had 1 shot kills, only frustration was watching 3 more spruce fly off while I try and dig out the 32 to reload
Same day as the last Spruce Grouse I ended up walking an old spur thinking more grouse then anything and had a 3X4 walk out 50 yds ahead, I quick switched and dropped him, got quite a kick out of taking small and big game with the same rifle, same day - still have that grouse in the freezer as it lost to backstraps.
I found the decibel level to be 22ish (as "accurately" measured by 2 sets of ears) but with a decidedly deeper "donk" (careful if you repeat that)!
So in conclusion, calipers showed it has a slightly larger throat then standard 308 ammo which led to it's stickyness in my savage but I am confident I could have made it work, it's cheaper then other adapters already and gave me a cheap chance to see how the small game load would work in my rifle (I have now bought reloading equipment for the sole purpose of downloading small game rounds). If you planned on plinking or target shooting, I think the pain in the butt factor would kick in soon with constantly extracting and reloading the adapter - but if you do, sand it shiny first! Much easier to find in the dirt then the factory gunblue and with one grouse I opened the H&R and as anyone who has one knows - it flings the shells WAAY out and I had to comb the ground for a while to find it.
Hope that helps (and that someone is still reading this)