What is the effect of varying the neck tension on accuracy?
Has anyone done comparisons?

Dr Tom wrote on May 18^th, 2013 at 11:04pm:


How would neck tension have an affect on a BS bullet ?.
  52-Shooter

Tim,  i have  tried several ways to get neck tension more constant with my 32-40 and a pinch of UniQ,   roll crimp was the standard with about .002..   shot well, but now still use .002, but after all are loaded with no roll crimp, I use my lee size die with the guts out and with the pill in the correct place, run the size die down on the neck for .08 to .10, seem to give less veration on the crono and quite  a few more hits on the rams,   see in a coupla weeks, Mike

52-S: Experts, please correct me if I'm wrong. Neck tension in BS is contact with the chamber walls, not the bullet. So, it's backwards from fixed. Neck compression? The case mouth is belled outward slightly to form a seal with the chamber. How much? Nooo idea. I'm still learning BS. My latest attempts with 25-20SS have very little bell. Maybe 0.002 over chamber neck diameter? Case length variation and the extent of the chamfer affect how tight it feels, but I'm almost always getting a seal. 

Tom

SSShooter, several years ago on the Shiloh site there was a fellow that had posted about using a .32-40 for silhouette.  He was using a very fast twist and heavy bullets.  If I remember correctly, the twist was something like 1:8! 

Chris.

When colo-native mentioned 'rams' (plural), thought he was talking silhouettes. 

Have a CH4 bushing-type neck tensioning die that I've tried on the 38cal. Have found it to be detrimental to accuracy when used at 0.400" & 0.401" on the outside of the loaded case neck. Am going to order up a set of 0.402" & 0.403" bushings to try. If nothing more, will result in a more perfectly round cartridge.

By no means do I claim to be an expert but here's the results I got Monday.  Rifle: 1874 Shiloh Sharps, Badger Barrel, 40-65 Winchester chamber, short leade.  Brass: Reformed WCC 67 45-70 fully fireformed, annealed and trimmed, etc.  Load: 65.2 +/- 0.1 gr Goex Olde Eynsford FF(g) drop tubed, 0.019 card wad, compressed 0.520 below case mouth.  Bullet: 420 gr Paul Jones 40001 with half thickness first driving band, cast 30:1. Lube: Emmerts.  Bullet was seated against the wad to set the case mouth mid way down the second driving band, leaving the second driving band about 0.050 off the lands.  10 shots were fired with slip fitted bullets, one fouler, wiped with RV antifreeze mix, at one of the ASSRA 200 yard iron sight targets at 200 meters.  The barrel was cleaned, the sights lowered 3 minutes to separate the groups, another 10 shots at the same target except these had been neck sized then expanded with a 409-411 plug and seated with the seating die set to just straighten the slight flare in the case mouth.  Again, one fouler, then an RV antifreeze wipe between shots.  All shots prone off crossed sticks, iron sights.  Without neck tension the group was about 8 inches, with neck tension about 2 inches.  Certainly not all that impressive for a schuetzen match but I was trying out my new powder and the theory that slip fit bullets might be more accurate in my BPCR rifle.  They weren't.  Your mileage will vary.

Dr. Tom, I believe you are correct.  The gentleman who suggested that I try slip fitting has a Browning BPCR 40-65 and did neck turn his brass to get it to chamber.  The neck on my chamber is nowhere near as tight as his.  My fireformed brass will easily accept a .410 bullet base.  My expander plug leaves the brass .411 at the mouth and .409 in the neck which results in a theoretical 0.001 interference fit.  I doubt that swaging my bullets in the bullet seating die has a beneficial effect but having them held concentric to the bore obviously does have a measurable effect on precision.  I had previously shown myself that any amount of crimping was detrimental to precision when shooting soft lead bullets, which is why I now only straighten the mouth to touch the driving band  

Another knowledgeable mentor suggested to me that my schuetzen rifle might work better if I flared the mouth of the case some to assist in sealing the chamber when breech seating.  I have yet to quantify the results but it did stop blowing soot back into the chamber and made seating subsequent cases much easier during the match.  I might try leaving the slight flare on the mouth of my BPCR rounds too if I can get them to chamber without too much effort.  That Sharps breech block is really intolerant of incompletely chambered rims.  That is my fault though as I had the finish reaming done to my specification and there is essentially zero headspace with my brass.