[m102404]

The A+ rubbers are great. The stock rubbers swell and bulge and will hang things up once they're out of shape.

I'm not sure of what part is cracking. If it's the front fake lugs of the nozzle then as said...they're mostly cosmetic. They usually break if there's a misfed BB or foreign object in there when the bolt slams home.

The nozzle doesn't have to retract...but it's not helping anything if it isn't. The fit of the rear of the nozzle in the "cylinder" part of the bolt should be as smooth and frictionless as possible...then use a light coat of grease for airseal. Light sand (i.e. green 3m or 1200 grit), clean, polish as needed. Try different o-rings to get the best fit. A proper fit will result in better gas efficiency especially since a lot of the gas is used to push the bolt back, especially so with heavier bolts.

If you've got a bolt buffer block in the stock tube that may stop it from locking back as well. They're good for taking up shock and making for a snappier cycling of things...but it may be interfering.

The bolt catches on the front left (nozzle facing away from you) corner of the bolt and the rear edge of the bolt catch. The leg of the bolt catch takes quite a beating and so does that corner of the bolt. If they're rounded then just square them off...the exact forward/back dimensions aren't critical. Some bolts have a harder piece fixed there.

If you pull your bolt back with an empty mag it should catch. If it doesn't press on the bottom half of the bolt catch and if it does work then the spring under the catch isn't strong enough to keep it in position. If it still doesn't catch then it may not be long enough (the OB type are shorter and for some bolts you need to file it down so the bolt doesn't hang up riding on it). You can also mod each mag a bit so the notch that sticks up to press on the bolt catch rides a bit higher...but I haven't had to do that much.

To help ease the possibility of the nozzles breaking in the cutouts of the chamber you can chamfer the edges of the chamber areas and the lugs. Taper them a bit so that they slip into position. Dry-fit the nozzel in the chamber to see how much play there is...some are really tight, others are loose. A slightly looser fit is better. You can also shave down the front to back edge of them a touch...the bolt assembly doesn't actually index on the forward edge of the nozzle.

If you've got an empty mag in (dry fire lock on) and the upper/lower closed up the bolt and nozzle should ride back and forth really easily. Aside from just a bit of a hang up on the hammer it should slide nicely. If it's binding tight in spots, drop the mag and try again. If it's nicer without the mag then you can adjust your mag release (try multiple mags first).

If it fires/locks back/runs smooth dry-firing it...but breaks/hangs up/doesn't lock back with BB's...then I'd look at the fit of the mags.

Re. NPAS....with the M4 and longer barrel lengths to get them reliably sub 400fps w/0.20's you're mostly choking off the system. To get them sub 350 the system is barely functional. These things really like to run wide open...downside are high FPS.

With all the WE M4's I've had work well...I haven't done much more than replace the hopup rubber (and in the old ones replace part#66). I've reverted a lot of "fixed up" guns though. Usually not worth the headaches.