BPCR Match Reamer Design Issues
There has been much cussing and dis-cussing concerning reamer design over the years I've been designing them. The first was designed in 1980 for an XP-100 used in the pistol silhouette game. I still have that JGS wildcat beauty based on RWS 5.6 x 50 brass. Can you say 7-TCU on steroids?
I've put this reamer primer together as a result of seeing too many friends spend good money, time and effort to have a top quality BPCR built only to have it all for naught due to an inappropriate chamber for top-notch match accuracy.
Most of the hubbub was and still is focused on the chamber's front-end: case-stop step-down, freebore and leade angle. For those of you not familiar with those terms, here is a picture as one is said to be worth a 1,000 words.
Reamer/Chamber Front-end Diagram
Much has also been made of chamber-body design. But, it is trivial in my opinion, as well as from experience, compared to the chamber's front-end. For the chamber body we just want a snug brass fit that does not interfere with round loading or case extraction and at the same time does not contribute to out-of-alignment issues when a round is chambered. There is, however, one area to focus on, especially for BPCR, and that is designing the chamber for the brass to be used given the range of original chamber dimensions and reloading dies for a given cartridge, can you say, no SAMMI spec? Simple, no? So, let's get to the meat of the matter.
The design parameters concerning front-end chamber specs focus on the three above mentioned chamber sections:
Case-stop Step-down: Angle, starting diameter & ending diameter
Freebore: Length and diameter, cylindrical or tapered
Leade angle: Angle and starting diameter, ending diameter is bore diameter
Let's address each design parameter in turn and then put the whole package together by addressing what it is we should be trying to accomplish with front-end reamer design.
First, let's discuss the case-stop step-down. It is the transition from the chamber's neck diameter down to either the freebore start or leade angle start. Not too much to discuss except that some BPCR reamers out there have only a case-stop step-down at a shallow angle that acts like a leade. It starts at the chamber-neck diameter and ends at the barrel's bore diameter; not my choice for a top performing match chamber as there is way to much slop and bullet bump-up that must be squeezed-down as the bullet hits the bore/groove diameters. From research to date, it seems that style of reamer was used to "butcher" the paper patch match chambers of original rifles and have somehow come to be accepted as match chambers in this day and age. That is unfortunate, to say the least. This discussion is focused on match chambers, not hunting or military; which have different optimization focuses. I believe the straight, shallow taper from full chamber-neck diameter down to barrel-bore diameter contributes to increased base distortion and contributes little to better bullet-to-bore alignment. And, since the bullet's base edge is the "steering" part of the bullet that can't possibly be an accuracy enhancer. For a hunting or military chamber this design might be appropriate though.
In my mind, minimizing the distance between the chamber's neck-diameter and the barrel's bore/groove diameters will help improve the bullet's base-edge symmetry because there will be a minimum of bullet bump-up that must be swaged down again, and never symmetrically, I might add, as the bullet enters the barrel's bore/groove. It seems we want the bullet to enter into the bore/groove diameters with minimum distortion, especially on the base edge. My current designs have a 45 degree angle from the chamber-neck diameter to the beginning of the leade angle. That typically means there is only about 0.023" of unsupported bullet before the leade angle begins. With 20-1 alloy my guess is that not much bump-up occurs in that narrow gap, a good thing for the bullet's base.
The second area of discussion concerning the chamber's front-end has to do with freebore. Freebore is typically a cylindrical, but sometimes tapered, section that is barrel-groove diameter or slightly larger. If there is freebore in the chamber design the back of it is where the case-stop step-down ends. The front of the freebore is where the leade angle starts. Most freebores are from about 0.050" all the way up to 0.500" long. Most are in the 0.150" range. For match cartridges that are a bit short on case capacity, like the 35-40 Maynard, a freebore can add a little more powder capacity due to being able to seat the bullet out further. But, at this time I'm inclined to say that a better strategy is to design a bullet that seats out into the bore for increased powder capacity. Some also call this "throating" the chamber and is often done after the barrel has been chambered, especially by bench rest shooters shooting jacketed bullets and smokeless powder. After working with various freebore configurations over the years I no longer included them in typical BPCR match chamber designs. That being said, it should also be stated that front-end chamber design and bullet design go hand-in-hand. Most of my current bullet designs are specified so that they seat substantially into the bore for enhanced bullet-to-bore alignment.
The final discussion point is about leade angle, a much debated design parameter. I will admit to going from 1/2 degree all the way to 3 degrees per side for grease-grooved and grease-grooveless bullets. Paper Patched chamber design is another discussion for now. After numerous reamers and much testing I've settled on 3 degrees per side, 6 degrees included to support the best possible bullet-to-bore alignment for the current crop of Paul Jones bullets I've been designing and Paul and David have been making molds for.
Now for the piece de resistance; addressing the whole front-end design package as a single entity by describing what it is we should be trying to accomplish with front-end reamer design. That design goal is; the best bullet-to-bore alignment possible with minimum bullet distortion especially on the base edge.
Some of you might ask, "How can we determine if a design accomplishes said goal?" Well, my friends, that is so simple you will fall off you computer chair when I tell you.
So, here it is. Make a dummy round (brass and bullet only) for your favorite load. Next, measure the bullet roll-out when the case neck is supported and the dial indicator is just behind the tip of the bullet. Write down the roll-out. Now, chamber the round and then carefully extract it. Measure the roll-out again. Is there any reduction in roll-out? If not, either your loading OAL is not what it should be or your chamber does not enhance bullet-to-bore alignment. For chambers and bullets currently in use, if a dummy round has its bullet purposely seated to create 10 thou roll-out; after chambering said round the roll-out will be reduced to between 2 and 3 thou. It's just that simple.
I guess one last issue should be discussed, chamber-neck diameter. For match chambers, "tight" necks seem to produce the best accuracy and they certainly are easier on brass due to reduced expansion and contraction every time the brass is subjected to the high pressures generated when the powder ignites. Current reamer designs allow for not neck-tuning the brass. But, slight neck-turning (just enough to remove high spots) to improve case-neck thickness consistency for more even bullet release can be done with no deleterious affects. The current design rule is simple: loaded round plus 1 to 1.5 thousandths clearance per side, 2 to 3 thousandths included. Also, the chamber neck's are cylindrical so the bullet has enhanced support as it starts its journey from case to target. One final issue to consider, the tighter the cartridge fit in the chamber-neck area, the less the bullet can bump-up as a result of its initial whack on the base from the rapid rise in pressure. From research to date, match chambers from the BPCR era were very tight and top marksmen were meticulous about their brass quality.
Happy pondering; think "Tight and well supported with enhanced concentricity from case to bore for our meticulously hand-cast bullets." Now that I think about it the true purpose of a BPCR match chamber is to deliver the bullet into the bore with minimum distortion and perfect alignment.