check the bolt gap by:
1. clear and safety the weapon, visually and physically confirm it's empty and remove any live ammunition from the immediate area.
2. allow the cocking lever and bolt carrier group to go fully forward and the rollers lock into place. you can "slap" the cocking handle downward if you like, it doesn't hurt anything if done sparingly.
3. refer to the picture to locate the bolt gap. this is a gap between the rear of the bolt head, and the front of the bolt carrier. this gap is functionally the "headspace" for roller-delay rifles.
3a. when viewed through the magazine well, you will see this:
3b. this represents the most forward position of the bolt head and the seating depth of the bolt into the trunnion - with the corresponding rollers against the angled shoulder of the locking piece and the recesses in the trunnion.
4. with a feeler gauge, insert it in this gap. start with 0.010" and move up from there. when you encounter resistance, back off one gauge, record that gauge's thickness (for example 0.014"), then repeat steps 1, 2, and 4.
5. if the number is consistent this is the "bolt gap". it should fall between 0.010" through 0.018".
5a. the larger the gap, the "less wear" there is, or more precisely, the less the locking piece is pushing on the rollers. either the rollers are over-sized, the wrong locking piece is used, or the rifle is extremely new. 0.020" is the upper end of this for a brand new rifle that has never been shot and is "tight". expect harder recoil and inconsistent ejection/extraction for about a hundred rounds or so.
5b. the smaller the gap, the "more wear" there is, or more precisely, the more the locking piece has to enter the trunnion to seat the rollers. this can mean the rollers are under-sized, the recesses in the trunnion are bulged (flat spots on receiver), or the wrong locking piece is used. a gap under 0.008" is marginal and can have hard ejection, excessive BCG velocity, rim tears, et c. a gap under 0.004" should not be fired as this implies excessive wear or insufficient delay resulting in possible damage to the receiver or bolt carrier or barrel. it is very difficult to have an out of battery detonation with roller-delay but it is possible with a very small or nonexistent gap and not allowing the bolt to fully seat forward.
5c. locking piece and roller size is functionally what determines "lock up" and "headspace". it is tuned to specific ammunition characteristics, barrel length, suppressed or not, the stock being used (buffer type, et c) and so forth. the G3 and clones are the most forgiving (and has the widest parts availability and are the easiest to work on imho) with the HK33 and MP5K's being much more finicky.
6. it is a pain to swap rollers but doable if you are mechanically inclined and can follow instructions precisely with some hand tools. it's not intended for field maintenance but is an armorer-level repair typically. if gap does not meet specification after going up or down 2 roller sizes (+/- 0.004") then the rifle is generally considered unsafe to use. extended roller sizes do exist, however these should be used with caution and is a band-aid on a problem. the best situation for an out-of-spec G3 is having one that is on the high side like 0.026" or something and coming down on roller size -0.006" to be barely within spec. as the rifle is shot it will "wear in" and normal rollers can be swapped in later in its life. using very over-sized rollers in a very worn rifle is very not advised.