The answer is the straightforward one. Brens on IS, piats on royal engineers. Piats are obviously better on engineers because they bleed less, have weaker stock weapons (so you lose less by giving them AT weapons), and most importantly, have a snare. As for which one uses MGs better, that comparison takes a little longer to lay out.
Obviously the idea is that engis are significantly cheaper than IS, bleed less, do not suffer from the out of cover penalty, lose DPS slower (since all the DPS is from the LMGs and not the stens), and can have 1 vickers + 2 brens. The thinking is good, but it just doesn't translate very well in game. Royal engi vet bonuses are absolutely terrible for LMGs. The vet 1 cover bonus (reload and cooldown) only applies to stens, so its close to absolutely useless. Vet 2 gives no combat bonuses, and vet 3 cant make up for the other two levels. I'm pretty sure royal engis are the ONLY squad in the game not to gain accuracy with vet, and that's absolutely massive. In the end, you have a squad thats cheaper than infantry sections (manpower cost and upkeep), but costs way more muni, walks way slower, can't heal itself, requires a lot of teching, and hardly performs better than sections do.
(A quick note stating that REs used to get buffed brens, but were recently changed to get the standard infantry section brens; this is actually what adamírcz is referring to).
ty for your answer and your guideness to this i was really confused what should i do,ofc i was testing them reading these forums but seems they are outedated,back to the topic if i correct get the meaning of what you say and my personal expirience so far from brits ,they are doomed at early stages in a 2vs2 and above game ,i mean with IS you have to be in max range to do damage and retreat asap as you see storm-pionners charging ,last thing ... is there anywere to see the dps in range from IS and other inf units in the game so i know where to stand in fight?
tyvm