I have one of the century rifles built up on a G3 kit - Hk internals, bolt, carrier and Port. barrel.  After sending it back to Century to have them fix the misaligned cocking tube and f'd up sights, it runs great.  I get 2-2.5 inches using hirtenburger surplus at 100 yards. 100% reliable after about 350 rounds.

Today, I thought that I would try out which combo of bolt/carrier produced the best groups. (I was fortunite enough to buy a couple of mint parts kits a few years back.)  All of them had passed the go/no-go gauges before in this rifle.  Heres what baffled me  -- with one combo the recoil is noticable heavier (on the level of my 7mm mag).  Any thoughts as to why?  Incidentially, it was the combo that shot the worst groups, but only by about 1/2 inch at 100 yards.

Thanks

Before you go buck wild and start swapping bolts/carriers, you need to think about roller sizes and B/C gap.  

The way a G-3 headspaces is determined by the roller size in relation to the face of the barrel.  No headspace gauge is going to tell you if the headspace is correct.  If the barrel is set up to bolt gap with a standard roller size, then when you install a bolt with over size rollers; you have increased the bolt/carrier gap on lock-up.  The standard working Bolt/carrier gap .008 to .012 between the two with the carrier locked against the bolt (action closed with the hammer down). By increasing the gap with an over size set of rollers, the bolt unlocks too soon and you get severe recoil.

FYI: The way the rollers work on a HK is that they make the bolt/carrier act as if they are heavier that the actual weight (think blow back action).  The force of the fired ammo forces back on the bolt, which forces on the roller, with the mass of the recoil spring and carrier weight keeping the rollers in the locked position. At a given point of back pressure force from the fired round, the bolt forces the roller back into the into the bolt (unlocking them from the trunnion and allows the bolt to unlock and be driven back with the carrier. The HK uses a buffer (in the butt stock to absorb normal recoil, but not that of an action opening up too soon due to the having the B/C gap-relationship out of spec.  Also, since the factory spec gap is designed around Nato ammo, by going with ammo that has a heaver bullet or stronger load, the action tends to open up too soon, which in turn, causes the rifle to have a stronger (above normal) recoil that the buffer will not tame.

By going with an excess B/C gap (oversize rollers), you have just lighten the overall geometry to keep the bolt closed under recoil, hence made the bolt unlock too soon during the action cycle.

Dano,

Thanks for the reply, but I still have no idea what you mean - I'm sure its my ignorance, not a problem with your explaination.

How doe one measure the 'gap'?  (Especially with the hammer down since this would mean to me that that rifle couldnt be broken down into trigger group, receiver and barrelled upper.)

I do think that youre correct about it being a headspace issue as cases fired using the worst recoiling combo have shoulders stretched to SAMMI maximium specs whereas the best accuracy and least recoil came from the combo in which the fired case only expands to .002 over SAMMI minimum specs.

www.cetmerifles.com/forum/viewtopic.php?t=120

Thanks Moto!  Exactly what I needed to understand what Dano was telling me