Whether you are building a Delayed Roller Blowback rifle, or buying one. It is important to know how to check cocking tube gap and bolt gap against each other. We all know that bolt gap is crucial to operation and safety of the DRB's, CETME or Hk. But, sometimes we fail to realize or remember that both work together. On a properly assembled or functioning rifle, as bolt gap decreases with normal wear, cocking tube gap decreases by the exact same amount. Many times, when buying a DRB that is either used or has been assembled improperly, the bolt carrier can be bottoming out on the cocking tube support, creating false bolt gap and doing damage to the rifle from the bolt carrier impacting the cocking tube support during firing. So, the buyer or builder can check the bolt gap and it appears to be fine, but if he didn't check the cocking tube gap, he can find out too late that all is not well. My advice is that when checking bolt gap the first thing that should be checked is proper cocking tube gap. It does no good to check bolt gap if the bolt carrier is bottoming out on the cocking tube support.
In the follwing pics, I try to show how the process works so it can be used by both builders and prospective buyers who may not understand all the crazy terms and actions we DRB nuts blabber on about.
First do a quick look to make sure that the charging lever is fully seated against the cocking tube, and is fully latched over the detent stub.
Next, Remove the magazine, and make sure the safety is up in the fire position. On a CETME the safety being on will prevent the bolt from being fully opened, on an Hk it doesn't matter.
Safety Fire T.jpg
Pull the bolt all the way back and lock it.
When you start to pull the charging handle, the lever should be loose and come out away from the rifle with no resistance to about 1/2 inch, about enough to get a finger under neath the lever to pull against it to unlock the bolt.
You should be able to lever the charging handle out to a full 90 degree position and feel the bolt unlock. The force necesary should take no more than 1 or two fingers, but it should be pretty stiff at first until the lever unlocks the bolt.
You should be able to pull the charging handle back with one hand. It's pretty heavy, ( 20 to 25 lbs.) but you should not feel any excessive dragging or feel any obstructiuons until the charging handle is all the way to the rear.
Lock the bolt to the rear by rotating the charging handle up into the detent notch.
Look to be sure that the chamber is empty.
Point the muzzle down and slap the lever down with an open hand. releasing the bolt and letting it slam closed.
Again check to be sure that charging handle is all the way forward, and that it is resting against the cocking tube and locked over the lever detent.
Pull the trigger to let the hammer fall against the bolt carrier. This is necessary later in the bolt gap test, as it makes certain that the bolt head is setting tight against the barrel breech, and that the bolt carrier is being held forward by the pressure of the hammer spring.
The next step is to make certain that you have the correct cocking tube gap. There are two methods used to do this. One is a cursory test or what has come to be known as the "dime" test, which is real handy when you are inspecting a rifle to purchase. The other test involves actually comparing the cocking tube gap to the bolt gap, making certain that both are ok. We will show the set up test first so that you understand the relationship between bolt gap and cocking tube gap a little better.
NOTE: Too little cocking tube gap will make the charging handle harder to unlock. Too much cocking tube gap will let the charging handle be easy to fold out , but will not let the bolt fully unlock, and will make pullling the bolt open extremely hard if not impossible. I note this so that you will not be confused between a lever that is hard to fold out and a bolt that won't unlock.
TOO LITTLE COCKING TUBE GAP = CHARGING HANDLE WILL NOT FOLD OUT EASILY
TOO MUCH COCKING TUBE GAP = BOLT WILL NOT UNLOCK, MAKING IT HARD OR IMPOSSIBLE TO PULL THE BOLT OPEN.
Make certain that the charging handle is pushed all the way forward,
CT Gap C.jpg
Note or make a mark on the cocking tube where the rear of the cocking tube suppport is resting.
CT Gap 1.jpg
I prefer to use a piece of masking tape as a reference point.
CT Gap 2.jpg
After marking the forward position, we need to see how far the cocking lever support moves. Which is done by lightly pulling out on the lever until it first reaches resistance. When doing this you will see the cocking tube support move slightly to the rear.
CT Gap 1.5.jpg
If the charging handle is new or near new, it should lever out about 1/2 inch from the cocking tube before it meets resistance.
CT Gap B.jpg
Mark the rear most position of the cocking tube support...
CT Gap 3.jpg
The distance between the front and rear positions is your cocking tube gap. That distance should be equal to your bolt gap plus .015". So on a rifle with .020" max bolt gap, the cocking tube gap should be .035". It can be as much as .030 more than bolt gap and still function correctly, but the minimum measurement is bolt gap plus .015". This extra tolerance allows for the cocking tube carrier to stay out of contact with the bolt head during firing, and to make sure it does so as the rifle wears and the bolt gap decreases. So even with .000 bolt gap, your rifle should still have at least .015" to .030" clearance between the cocking tube support and bolt carrier. ON THIS PARTICULAR RIFLE THE BOLT GAP IS .018" SO THE COCKING TUBE GAP IS .018" + .015" = .033"
CT Gap 5.jpg
Now that we know we have cocking tube gap, we know that the bolt carrier is not resting on the cocking tube support, causing us any interference, and we can continue on to check bolt gap, by inserting our feeler guage between the bolt head and the carrier. Again, bolt gap on this rifle is .018", so we know our bolt gap is .018" and we know our cocking tube gap was .033" so this rifle is good to go.
CT VS BOLT GAP.jpg
NOW THE DIME TEST.......
Cocking tube gap can be checked fairly easily when inspecting a rifle to buy, by doing the dime test. Most US dimes are between .035" and .050" thick. So if you determine that the rifle you are considering buying has good bolt gap, and no ground bolt head, you can compare the amount of movement on the cocking tube support to the thickness of a dime, you should fall into the acceptable catagory for cocking tube gap if it is no less than the thickness of a dime and the charging handle moves out to around 1/2 inch away from the cocking tube.
CT Gap 6.jpg
CT Gap 7.jpg
CT Gap A.jpg
When doing a build, the same methods can be used to determine the correct positional length of the cocking tube by barrell pressing and setting gap and then positioning the cocking tube to add the .015" to your set bolt gap.
HOW MUCH IS TOO MUCH YOU ASK?????
When looking at buying or when trying to figure out why your DRB rifle is hard to unlock, it's good to know when too much money is a bad thing....
If your cocking tube gap gets around .0785" or 2mm, you are getting close to the point that the rifle will not properly unlock. If the cocking handle comes out farther than about 1 inch from the cocking tube it could indicate that the charging handle cam is worn or that you have too much cocking tube gap.
A worn charging handle will show up as a nearly good cocking tube gap, but the handle comes out near an inch, once it gets worn to that point it is likely to not fully unlock the bolt head. It will cam over and be extremely hard or even impossible to pull back.
CT Gap D.jpg
If the cocking tube gap exceeds a maximum of .0785" (or 2mm) the same difficulty in unlocking the rifle will occur....
CT Gap F.jpg
So, in addition to your dime, take along a nickel when you are looking to buy, a US nickel is about 2mm thick...That fifteen cents in your pocket might keep the paper money in your wallet.....or keep a new purchase off of the bench.