HK416 strange accuracy issue

Hi all.
I have lately been using my HK416 in the shed with a Nerf digital target, which is pretty good fun actually.
My HK416 has a 7.5mm barrel, stock standard gearbox, using LDT milkies, 10m range to target.
No hopup being used.

So there are 7 target hits per round.
First the first 4 rounds, so 28 shots total, the blaster accuracy is pretty good, can get bulls eye shots fairly often.
But, for the 5th round and after, the accuracy turns to crap, the overall spread is twice as bad.
This same situation is 100% repeatable every time (I do total of 10 rounds per session).
Would love to hear from people as to what might be causing this to happen. Preferably would like to have all the rounds reasonably accurate, without it suddenly stuffing up after 28 shots or so. Cheers !

Sometimes I find that the gearbox can get a bit hot and the stock o rings and spring can be affected by this heat. I would suggest a new cylinder head and a green o ring and a slightly larger spring. I hope this helps

Ok Sam, thanks for this info. Heating of gearbox parts could make sense.
Actually, now I think of it, I wonder if initially my fps is lower, for better accuracy, then goes a bit higher after awhile, stuffing up the accuracy ?
Some kind of air sealing issue perhaps ?
Unfortunately, I dont have a chrono to measure with.

Should mention, it is an earlier 2.5 version HK416, so no built in hopup.
And shooting single shots only, not full auto.

Yeah I defs suggest eliminating air seals and the 100 percent cylinder. This will increase consistency therefore giving you hopefully more accuracy