First off let me say, I am not a gunsmith. I am even less an expert on delayed roller blowback (DRB) rifles like the CETME or an HK 93. However, let me say that this post from military firearms.com does an OUTSTANDING job of describing what is going on inside your rifle.
If you’re not interested in knowing how it works and just want to know how shoot, my article will tell you what you need to know to ensure your CETME or G3 has proper bolt gap.
However, I would highly recommend you read the military firearm post by bladeworks123, because the more you know about your rifle the easier you will be able to keep it running.
Bolt Gap is Not Headspace
You need to know that there is a difference in bolt gap and headspace. Headspace is the distance measured from the part of the chamber that stops forward motion of the cartridge to the face of the bolt. This is important. If the headspace is incorrect, ammunition will not chamber correctly. Alternatively, it may rupture and cause damage to the firearm or the shooter.
In locked bolt guns like an AR-15 you can buy gauges that are machined to look like cartridges. The gun should lock up on a “go gauge” and fail to lock up on a “no-go” gauge. When a DRB rifle barrel is pressed into the receiver, it is not pressed to the face of the bolt. It is pushed even further so that the rollers in the bolt head are pushed against the locking piece. The locking piece is putting pressure on the bolt carrier creating a gap the bolt head and the bolt carrier.
Bolt Gap is Critical
This gap is critical. It is the point that the action is about to open. Consequently, it is the measurement that allows the rifle to function correctly. If the barrel is pushed in too far the action will open too soon. Alternatively, if the barrel is not pressed in far enough, the rollers will have to travel too far to unlock the gun. Since the back pressure from the gun’s firing may not last the full trip the gun probably won’t cycle. If it is extremely short than then portions of the neck of the cartridge will be unsupported and may rupture.
In a DRB a go or no-go gauge will both seat in the firearm. There is no true way to measure headspace in a rifle such as this, but the bolt gap measurement achieves the same result and is a more accurate measure of what you are trying to check.
Here is the procedure:
- First, remove any magazine and check that the chamber is empty.
- Perform the HK slap by pulling the charging handle to the rear and locking it. Then, slap the charging handle down with the palm of your hand sending the charging handle forward using spring pressure only. This is important to get a true measurement. Riding the handle forward may cause it to bind at the end of the cycle.
- Pull the trigger so the hammer is down.
- Invert the firearm so the empty magazine well is pointing up.
- Next, open the feeler gauge up to .5mm. Try to slide it into the very slight gap between the rear surface of the bolt head and the front surface of the bolt carrier.
- The .5mm measurement is the maximum the gap should be.
- Open the feeler gauges to the .25mm leaf and try to fit that between the bolt head and carrier. The feeler gauge should fit without forcing it. Additionally, if the gap is smaller than .25mm the likelihood is that the gun will have problems with extraction / ejection. In fact, it may not run at all.
Buy the Right Gauges
You should buy a feeler gauge set that has several leaf sizes between .25mm and .5mm. This will let you get a good idea of the bolt gap. Checking this measurement regularly will let you track accurately the bolt group wear.
If the bolt gap is not between .25mm and .5mm, there are oversize and undersize rollers manufactured. Next, you can swap them with the stock rollers to either increase or decrease the bolt gap. Finally, if the gap is substantially out of spec, changing the barrel or reinstalling the barrel either further in or out may be required.
As a side note (I am not paid to say this)
I buy my rollers from RTG International Surplus Parts in Arizona
And for barrel work, I use wither Dave’s Gun Shop in Cookeville TN (I’ve been in firearms classes with Terry their gunsmith – great bunch of guys)
Or Lock Stock and Barrel in Franklin TN (sorry no website)