https://www.tngun.com/cetme-bolt-gap/
First off let me say, I am not a gunsmith, and I am even less an expert on delayed roller blowback 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 your carburetor works and just want to know how to stick in the key and drive, then my article will tell you what you need to know to ensure your CETME or G3 has proper bolt gap. BUT, 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.
First off, 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, because if the headspace is incorrect, ammunition will not chamber correctly, or 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, and the gun should lock up on a “go gauge” and fail to lock up on a “no-go” gauge. When a delayed roller blow back (DRB) rifle barrel is pressed into the receiver, it is not pressed to the face of the bolt, but even further so that the rollers in the bolt head are pushed against the locking piece, and the locking piece is putting pressure on the bolt carrier creating a gap the bolt head and the bolt carrier.
This gap is critical, because it is at this point that the action is about to open, and this measurement allows the rifle to function correctly. If the barrel is pushed in too far the action will open too soon. If the barrel is not pressed in far enough, the rollers will have to travel too far to unlock the gun, and 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:
Remove any magazine and check that the chamber is empty.
Perform the HK slap dance 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.
Open the feeler gauge up to .5mm and try to slide it into the very slight gap between the rear surface
First off let me say, I am not a gunsmith, and I am even less an expert on delayed roller blowback 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 your carburetor works and just want to know how to stick in the key and drive, then my article will tell you what you need to know to ensure your CETME or G3 has proper bolt gap. BUT, 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.
First off, 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, because if the headspace is incorrect, ammunition will not chamber correctly, or 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, and the gun should lock up on a “go gauge” and fail to lock up on a “no-go” gauge. When a delayed roller blow back (DRB) rifle barrel is pressed into the receiver, it is not pressed to the face of the bolt, but even further so that the rollers in the bolt head are pushed against the locking piece, and the locking piece is putting pressure on the bolt carrier creating a gap the bolt head and the bolt carrier.
This gap is critical, because it is at this point that the action is about to open, and this measurement allows the rifle to function correctly. If the barrel is pushed in too far the action will open too soon. If the barrel is not pressed in far enough, the rollers will have to travel too far to unlock the gun, and 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:
Remove any magazine and check that the chamber is empty.
Perform the HK slap dance 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.
Open the feeler gauge up to .5mm and try to slide it into the very slight gap between the rear surface
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