Author Topic: Chamber Lead In  (Read 2474 times)

Bob La Londe

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Chamber Lead In
« on: November 24, 2016, 07:31:31 AM »
I am curious if any experiments have been done with chamber shape and lead in to optimize performance.  Lots of barrel liners are available that I think would make great barrels for home built air guns, but they all assume you will be using a chamber reamer for the powder caliber you are using.  With an airgun that is not usually the case.  (pun intended) 

Do you size it so the pellet already engages the rifling when seated in the chamber?  Just short?  Barely touching?  With a smooth lead to start building speed before it engages? 

Is it going to need to be different for pellets and bullets? 

Could this be in part why some pellets perform better than others in a particular gun?  Even when power is adjusted to get them in their "ideal" speed range. 

Then there is the transition itself.  I don't think the low velocity and pellet hardness isn't going to cause throat erosion like it does with a powder burner.  I would think a sharper start will cause lead chips (cutting the projectile to fit the rifling) while a more tapered start will be more likely to cause the pellet or bullet to deform to match the rifling.  Should the rifling taper into its full form or should it be a fairly sharp start? 

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Re: Chamber Lead In
« Reply #1 on: November 24, 2016, 08:11:30 AM »
This is a very interesting subject to cover, and I'm looking forward to others' input.

From a powder burner standpoint, the most accurate setup for most calibers is a few thousands off the lands. For example, the most accurate rifle I ever owned, was an Ackley Improved 22-250. The bullets were seated .002 away from the start of the full-sized lands. The inlet taper (throat) was a scant .025 thousands in length, and of the same ogive as the bullet. This tends to minimize throat erosion, as the bullet seats (starts obturating thus less amount of escaping gases) before the tail end leaves the case (tight neck scenario). Obviously, you don't have this much control in an airgun.

Probably the most important attribute is seating the pellet concentrically with the bore. Thinking about this on the fly, a smooth-twist barrel should have a slight advantage over a standard barrel with a short throat, however it is done.

Along this same line, was all of the research in the mid to late 70s on gain-twist barrels. For those that don't know, a gain-twist barrel may start out having a twist of 18:1, but increases over the length of the barrel, and may be as much as 10:1 at the muzzle. It turns out, that throat design is more important, than the type of rifling (square or round cut and/or number of lands). I believe this is what Bob was alluding to.

As I said, this is an very interesting subject. Nonetheless, I still wonder just how much more accurate an airgun can be made? I don't own one, but I've seen air rifles which almost literally cut the same hole shot after shot. Maybe this thread will help shed some light on the subject.

PS: You do come up with some really interesting things to talk about, Bob!
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Re: Chamber Lead In
« Reply #2 on: November 24, 2016, 09:35:27 AM »
I don't think the chamber and leade makes a huge diference with pellets, but it certainly does with bullets, in particular regarding the chambering effort.... Pellets have the skirt quite a bit larger than the head, as much as 0.008-0.010" in some brands.... If you push a pellet through a barrel, you may find only slight marks on the head but deep marks on the skirt.... The skirt is designed to collapse on loading, and then expand again (obturate) on firing to seal in the barrel.... IMO, as long as the pellet chambers easily and straight, the chamber and leade are fine.... Since the bearing area on most pellets (EunJins being an obvious exception) are narrow, chambering difficulty isn't usually an issue.... I think a smooth chamber past the barrel port, and then a steep taper on the end of the rifling (30-45*) is probably all you need....

Bullets are another matter entirely.... Common PB bullets usually have 3 or more bands that ride the rifling, with lube grooves between.... On my latest NOE designs of BBT's I only have two contact points with the lands; the back of the nose ogive, and a single driving band at the rear.... The area in between is a couple of thou smaller than the land diameter, so at least in theory doesn't touch any part of the barrel.... I say in theory because if the bullet drops oversize and then you size it, the lands may just touch the recessed center section, although they would still have very little drag.... A chamber for a bullet needs to have two distinct parts, a parallel chamber, and a tapered leade that transitions from that to the full depth rifling.... The parallel chamber is at, or a thou or so larger than, the groove diameter of the barrel.... It holds the bullet straight in line with the bore to insure that it enters it aligned properly.... The bullet should be sized so that it slides perfectly in that portion of the barrel.... If it is too large, it will be hard to chamber, and if the bullet is too small, it may not be straight when it enters the leade.... The parallel chamber should be beyond the barrel port in a PCP.... The chamber should NOT be used as a sizer, that is not it's purpose....

The leade is much more subjective, and I have only used two designs.... The chamber reamers I have made myself have a taper that is 2* per side.... I purchased one from Sean Pero, and he uses only 1* per side, so the leade is twice as long.... There is NO question that you can feel the difference when you chamber a bullet, it chambers easier and more smoothly with Sean's reamer than mine, and the shallower taper is also more forgiving of different bullet lengths as well.... The reamer consists of a three sections.... a pilot section that slides inside the lands of the barrel and can rotate without dragging or damaging them, the tapered portion that cuts the leade, and the a parallel portion that cuts the chamber.... A perfectly made reamer might have closer tolerances, but I usually have the pilot 0.002" smaller than the lands and the chamber 0.002" larger than the groove.... I am not a good enough machinist to have smaller tolerances and not damage things, so I err on the side of safety.... The pilot section of the reamer has no cutting edge, it is just circular.... Both the leade and the chamber have a one or more cutting edges.... My reamers have only one, Sean's have up to six.... Here is one of my reamers....

You can see the pilot at the front, immediately behind that is the tapered part that cuts the leade, and then the parallel part that cuts the chamber.... I mount the barrel in the chuck (ideally it should be indexed off the bore), and the reamer in the tailstock chuck and run the lathe at very low speed (under 100 rpm) with lots of cutting oil in the groove, which is at the top to hold the oil and chips.... I remove the reamer frequently to clean it.... and yes, I have had one stick in the barrel and break off and had to shorten the barrel....  ::) .... Here is one of Sean's reamers for comparison.... Note the undercuts behind the pilot, and another one behind the chamber, something I will add when I have to make another one.... You may also be able to see the shallower taper compared to mine....

The depth that you run the reamer in determines the length of the chamber and the location of the leade.... As I said, the back of the bullet (and indeed the skirt of a pellet) should sit in the parallel part of the chamber.... The nose of the bullet (or the head band of a pellet) should sit somewhere along the leade.... Shorter bullets will not be as deeply engaged in the rifling as longer ones, so will be slightly easier to chamber.... although I have found that as long as the bullet is not more than 0.001" larger than the groove diameter (which means it is not tight in my chamber) that I can chamber a bullet without too much force even if it is longer than the leade and seats into the full depth of the rifling.... Even the 2* leade is capable of compressing the lead and allowing the bullet to be seated to full depth, if there is only one band contacting the rifling.... If the bullet has multiple bands, and the leade is too far back for the length of the bullet, then it gets progressively harder to chamber.... If you can't shove the bolt forward without thumping the bolt hand or hurting your hand, the bullet is too big, the chamber too short, or the leade taper not shallow enough....

In terms of what actual dimensions do to the accuracy, I have no idea.... I hope that after we close the Motel in 4 more years I will have enough time in the summer to get out and shoot enough to try and find out such things.... At the moment all I can do is give you the theory, my practical experience in making the chamber and leade, and hope you have the time to find out for yourself.... Fortunately, Forums such as this enable us to share our results, and I hope that some of our more experienced bullet shooters can help answer the OP's question better.... BTW, I have found that providing the chamber for a bullet is not so long that the front head band of a pellet is not engaged in the leade, that a bullet chamber can work just fine for pellets.... The reverse, of course, is not necessarily true, and even a bullet of the correct diameter for the barrel may be VERY hard to chamber if the leade is not in the right place, ie the chamber is too short....

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Re: Chamber Lead In
« Reply #3 on: November 24, 2016, 09:47:35 AM »
Bob, Thanks for a very informative post. I have been thinking about this very subject for shooting .25 bullets in my Mrod.

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