Chambers for Bullets

There seems to be some confusion about what the chamber in a barrel should be like for bullets, and how it differs from one strictly for pellets…. Here is a rough sketch of how they compare….

In many airguns, intended to use pellets, very little attention is often paid to the chamber…. In a PCP, or any gun that has a bottom port for the air, quite often the rifling starts behind or at the port, while in others it may start just at the front edge of the port…. Very often there is just a simple bevel, maybe 30* or 45* just so that the edge of the rifling isn’t a sharp corner…. This is because our pellets are very forgiving, because they have very narrow bearing areas, just at the back of the head and the back of the skirt…. and so it is easy for the bolt to push them past the port and into the rifling…. The hollow skirt is made oversize (ie larger than the groove size of the barrel), while the head can be anything from just touching the lands to nearly groove diameter…. None of this really matters, the bolt can easily push the pellet past the port, and it gets consistently (for the most part) loaded straight in the barrel with relatively little force….

With bullets, they have at least two (and often more) relatively wide areas (drive bands) that are at, or slightly over, the groove diameter of the barrel…. usually with “grease grooves” in between, or in the case of my NOE Bob’s Boattails, a reduced diameter midbody that just missed the lands of the rifling…. These smaller diameter sections reduce the friction as the bullet moves through the barrel…. Most shooters find that when the bands are 0.0005″-0.001″ over groove diameter the bullet shoots the most accurately…. If the diameter of these bands is less than the groove diameter, air can leak past, robbing power (and often leading to poor accuracy)…. If the diameter is much larger than the groove diameter, it take a huge amount of force to even chamber the bullet, and the high friction robs power…. If the bullet is too big, it can be impossible to chamber…. or even stick in the barrel when fired…. So firstly, the bullet must have the correct OD to fit the grooves snugly, but not too tight…. If the bullet, as cast, is too large, you need to “size” it by pushing it through a sizing die (basically just a hole) of the right size to squeeze the bullet down to the groove diameter, or within about 0.001″ larger (in most cases)…. Whereas pellets have a skirt that balloons outwards to seal in the bore, bullets have to start out at the right diameter….

Once you have a bullet that is the correct diameter, you may still find it hard to chamber…. The ideal chamber for a bullet, IMO, is one where there is a parallel section of exactly the same diameter as the groove diameter of the barrel (or maybe 0.001″ larger)…. and then a tapered area, called the leade, where the barrel transitions from no rifling to the full depth rifling of the bore…. Different guys like different tapers, but between 1* and 2* per side seems to be about right…. The shallower the angle, the easier it is to chamber the bullet, because is it easier for the rifling lands to compress the lead in the bands as the bullet slides forward…. To give you an idea of what that means, if the land diameter is 0.006″ smaller than the groove diameter, a 2* taper would be about 1/10″ long…. This leade should be positioned so that the nose of the bullet is partially engaged along the tapered portion, while the rear driving band of the bullet is still in the parallel chamber…. Long bullets therefore need a longer chamber so that the base of the bullet is ahead of the port…. or in the case of an axial port like a Condor, when the bolt is closed fully…. Fortunately, this is pretty forgiving in most cases, and having a longer bullet nearly engage the rifling fully, while a short one barely touches the leade, generally works just fine…. When the bullet is fired, the driving bands move forward, and the tapered leade presses the rifling marks gradually deeper, until all the bands are fully engaged in the full depth rifling of the bore….

Fortunately, since most bullets that would be shot out a barrel intended for pellets, are pretty short, we can usually have a chamber that will work for both…. If you compare the distance from the base of a pellet to the head band…. to the distance from the base of a bullet to the front driving band (or in the case of my NOE BBTs the back of the nose)…. they are pretty close (for bullets suitable for use in pellet barrels…. Therefore, by putting the leade in the right place, both pellets and bullets will chamber with the head/nose/front driving band somewhere along the leade…. while the skirt/base sits in the parallel chamber…. Chambers are best cut with a special chamber reamer, but they are almost impossible to find in airgun calibers…. If you need a chamber recut in your barrel because you want to shoot bullets from a pellet barrel, you may need to find a machine shop or gunsmith to do that for you…. I am pretty sure that if you contact Jim or Travis at W.A.R. they will either be able to help, or point you in the right direction…. As the number of shooters that want to use bullets increases, I think you will see this service being offered more and more…. Some home machinists/tuners have done this by hand, and maybe they will chime in with their techniques…. I would welcome that in this thread….

Bob