Yakov,
Don't make the mistake in operation theory that many make and think a Gas Delay Blow Back Action is ever "locked." It's not, the very nature of being a "blow back" design which primarily operates from inertia precludes it from being a locked design as we know it. If you are trying to design a hybrid system then you might have something that would be novel and useful.
The gas is a "retarding" mechanism, just as rollers, links, sliding blocks, etc. are retarding mechanisms on other delayed blow back systems. Not to be confused with short or long "recoil operated" systems which are "locked." The locking refers to the cartridge being locked into the chamber, it has no bearing as to whether or not that chamber may be on a "sled" recoiling at the time. Locking is very important with high pressure cartridges, not so much with small scale or low pressure cartridges. For instance the VG1-5 as well as the H&K roller delay blow back designs use fluted chambers or necks to create a gas film around the cartridge to allow it to move before the pressure has subsided. Without that gas "lubrication" the case would obturate and seize against the walls. The result is usually a ruptured or separated case.
To retard means to diminish or reduce, that's all. The VG1-5 is the only rifle or sub-rifle cartridge that was ever produced using that system. You need to also remember the gun needed the steel cases they had turned to as a alternative material to operate correctly, the brass cases would obturate too much and grab the walls. The current H&K battle rifles have gone to more flute length and the delay roller action actually slows the head down to a crawl while accelerating the rear of the group to keep the inertia necessary to extract the cartridge and cycle the action. There are three pistols that made it to production status including a Chinese design that is rather large and clunky.
I have a PSP, a P7 and a GB. I was once on the quest for an alternative pistol design and have played with them quite a bit. Note these pistols are all 9X19, there is a reason for that. H&K produced a few P7M10s in .40S&W and there was the .45ACP version called the P7M7 (maybe 6 or 7 were produced). There were also some non-gas delay P7 versions in smaller calibers that were pure blow back, but not germane to this discussion. The problem with the larger calibers is that the bullet mass required an increasingly heavier slide. The P7M10 was nicknamed "the whale." Even the GB is big and a bit clunky, that’s primarily because of the large multiple ports, their position and the large “chamber” and “piston” created with the barrel and surrounding chamber. The larger volume doesn’t have enough gas to fill it quickly enough and the ports are further up the barrel. The increased slide size gives you mass that is balance with the gas.
Always remember all of those pistols as well as the VG1-5 have the cases being extracted from the instant the inertia of the system is overcome. The gas is just an aid or an alternative to help you reduce the amount of mass you have to put behind the case. They are still blow back actions, not locked. The 9x19 is a very strong pistol case relative to the bullet size and weight. It can operate in pressure ranges that would rupture other cases. Modern military rifle cartridges operate in the range of 50,000-62,000 psi this includes 5.56x45, 5.45x39, 7.62x39, 7.62x51 or even the 5.7x28 P90 cartridge. The 9X19 even at the highest allowable NATO pressure is at 36,500 psi, the SAAMI +P is 38,500.
Unless you are going to design weapon that will stretch brass a’ la the HK rifles or like a lot of the machine guns that are extracting while under pressure you are going to need to lock the cartridge until the pressure has attenuated enough.
Best of luck to you,
Mako