Steyr H5 SA shot strings

[Steyr]

I have a 2 yr old Steyr H5 scout. recently noticed that the first shot I take with a new magazine always seems to be slightly lower than the rest of the shots - about 3/4 inch at 25yds (I don't have a tripod or means of fixing it solid, I shoot sitting down with a shooting bag draped over the back of a chair). So decided to chrony it. Shooting JSB jumbo heavy 18.13g. I have 3 magazines:

Magazine 1
886.3
886.3
888.4
889.5
895.8
mean 889.26
sd 3.9
var 15.27

Magazine 2
885.3
890.5
892.6
888.4
888.4
mean 889.04
sd 2.7
var 7.40

Magazine 3
888.4
891.6
890.5
889.5
895.8
mean 891.16
sd 2.85
var 8.13

It seems the first shot fps is lower than the other shots but not massively so and I wouldn't thought a few fps would cause a marked drop - is it my shooting, something else? really puzzled here.

 

[parallax]

I'm not very familiar with the Steyr line, does that gun have a regulator? The first shot on my .22 Wildcat is around 30 fps low and then back up to normal for the next shots. Have read that the regulator can leak down a bit during storage which results in a lower 1st shot MV.

 

[The_one]

Doesn't this rifle use the magazine as the chamber for each round instead of chambering into the actual barrel?
Could it be that something's wrong with the first chamber of the mags, or that the first chamber isn't as well sealed as the subsequent chambers because less of the actual mag is in the rifle when firing the first round? Are there oring seals in the breach area in front of and behind the mag body so that it is sealed?