Just bought the m&p 15 pistol (ar-15) with the 7-1/2” barrel. Went to the range today tried 6 different types of ammo, 55 grain to 77 grain. Shooting slow and easy from a bench at 50 yards with a 3-9x42 sig sauer scope testing for accuracy. Shot about 200 rounds, and averaged 2 - 4 inch groups, again at 50 yards. To me this is unacceptable. Any one else with similar experience, what should I expect?

Why put a scope on a 7 1/2” barrel? What is the purpose for adding the extra weight and bulk?

I would not expect the same amount of accuracy out of an AR pistol as I would a carbine or rifle.

From a strictly MECHANICAL standpoint an AR pistol should be able to group in the same range as it's parent rifle.

From a SHOOTABILITY standpoint it's largely a matter of the shooters ability and knowledge of shooting the AR pistol vs rifle. It does have certain quirks and takes some work.

Shooting from a solid rest, concentrating on the fundamentals, is the start.

From shootability standpoint, I agree. The carbine, or even a SBR with a proper stock will be easier to shoot accurately than a pistol with the brace (unless you shoulder the brace like a stock).

Mechanically, I am not sure. The 7.5" barrel with a 1:7" twist will only give the bullet a little over 1 full rotation, where a 16" barrel will give the bullet two full rotations within the barrel to stabilize the round. Less velocity from the shorter barrel as well. Based on that, I would not expect the same level of accuracy.

2"+ from a pistol pistol at 50 yards would be pretty good. Why would the AR platform be that much more accurate?

SBRs and AR/AK pistols really aren't for long range shooting. The bullet will fly hundreds of yards, but hard to be precise with them.

Velocity greatly influences accuracy at distances. Barrel length influences velocity.

I dont own an AR pistol nor have I ever shot one. This made me curious though and watched some video on it. Shooter was also at 50 yards, red dot/no scope but with a 10” barrel. Shooting off a table. And looked like similar results.
Not knowing how his set up, ammo (reload or new) and skill set compares to the OP the results probably dont mean much. But i have to wonder if 2-4” groups with a 7” barrel are respectable at that distance, without knowing more data?