They say that straight walled cartridges do not have to trim the brass,is this true for rifle brass also.
I'm curious because if the case is to long wont it cause to much pressure??
I'll share my thoughts based on 47 years of loading.
I have always checked or trimmer rifle and pistol cases based on the Load Manual Specifications. All Cases.
However, early on, many years ago, I loaded 1000 rounds of 9X19 Luger. I had used a Lathe Type Case Length Trimmer. As it turned out, about 10% of them (100+) didn't fire because of a light primer strike. Apparently, the setting on the Trimmer had shifted and I had trimmed them a whisker too short. That was when I stopped loading 9mm and started buying new. The cost savings wasn't worth the headache. Also in recent years, I started using the Lee Case Length Trimmer System. Consistent results first time, every time.
Straight wall or bottle neck doesn't matter. If the case has a shoulder or a rim, case length is critical. The danger from high chamber pressure comes from the risk of the neck extending too far out of the dedicated chamber or into the rifling. Having even a fraction of the case neck between the bullet and the rifling is extremely hazardous.
Having a fraction of extra space in the case is a moot point. The OAL (if properly set) has already determined the space (between the bullet's base and the base of the shell case) within the case regardless of the length of the neck. Follow my thoughts? And that space will vary anyway if you load a light bullet vs a heavier bullet. Always follow the AOL recommendations for the bullet being loaded. Always check the case length and trim accordingly.
As for the origin of your question. "I believe".
Only within the the past year or so have I heard anything about "not being concerned with case length". They were talking about semi-auto pistol cases and this
only applies to Rimless, Straight Wall Cases like 380 auto, 9mm auto, 45 acp, etc. That is because those cartridges get their head space by seating on the mouth of the case (not the rim or shoulder). If your OAL is correct, the only danger would be to rupture at the rim of the case if it is protruding outside the chamber.
I don't concern myself too much with rimless, straight wall semi-auto pistol cases. (my main concern is to not trim them too short) I do check from time to time to make sure they don't exceed the Max. Case length. This is seldom a problem unless you are shooting extra hot loads or your pistol's chamber is over size. With standard loads and a quality pistol, the case expansion & growth should be minimal.
Everything else gets checked and held within specifications.
I hope this helps.