Friday, December 17, 2010

Results from extensive testing, in pursuit of accuracy.

Working on accuracy in my 357 magnum Taurus 66, 6 inch barrel is great fun and an excellent hobby.  My goal is to obtain the best accuracy possible with cast bullets, or boolits in some circles.  Let's get started.

Accuracy can be achieved in all the moulds tested at a distance of 15 yards and less.  The big news to me is that some bullets that are accurate at 15 yards completely lose it at 25 yards.  On the other hand, everything that is accurate at 25 is extremely accurate at 15, that I did expect.

The loads and bullets listed are all capable of 1.5 inch groups or less at 25 yards.  Everything else didn’t group at all and won’t be the focus of this update.

The most accurate rounds to date are all using 357 brass as the length is critical.  The alloy for each bullet on the most-accurate list is 98% lead and 2% tin (more about alloys later):

1 - 358-429 Keith from NOE Moulds, .360 diameter at 168 grains over 4.9 grains of HP-38 and OAL of 1.620.  Groups of 1 inch or less are possible with this combination, with a best of .6 inch.  This bullet is big and may not fit in all revolvers.  It almost sticks out of the cylinder.  This is barely a +P load and was pan lubed with Darr lube.  Lyman lists 4.6 grains as the max standard pressure charge for 38 Special.

2 - 358-158-SWC tumble lube from Lee, unsized (roughly .3595) at 158 grains over 3.5 grains of HP-38 and OAL of 1.60.  Groups of 1 inch are possible with a best of .7 inch.  This is crimped into the second groove to get some length and is a 38 Special load.

3 - 358-158-SWC tumble lube from Lee (same as above), this time over 5.1 grains of HP-38 and the same 1.60 OAL.  Groups from 1 to 1.5 inches are possible with this light 357 load.

3 – 358-148 WC tumble lube from Lee, unsized (around .3595) at 148 grains over 3.5 grains of HP-38 and OAL of 1.40.  This is a tie for third best with groups from 1 to 1.5 inches and is a 38 special load.

Key factors to achieving tight groups, in the order of importance, at least in my view, are:

1 – Bullet design with good throat fit and the correct OAL.  Best accuracy is achieved with designs that help to center and align the boolit with the barrel, sometimes this requires a long OAL.  Not all revolvers will work well with these exact OAL’s, however keeping the bullet nose approx .03 inch from the front of the cylinder can sometimes help.  The Keith is actually .01 inch from the face, not much, and it’s accurate.

2 – Next in importance is the alloy.  This list is short.  The most accurate is BHN 7-8, a little bigger grouping with BHN 11-12, and even bigger grouping with BHN of 16 and greater.

3 – Last in the critical tuning controls is diameter.  Another short list, the most accurate is .360 followed by .359 and then .358.

Powder is key and HP-38 works well with lead.  I expect other powders may work as well but haven’t verified or tested very many.  I'll also be testing more with 700X which is another proven favorite.

Added later:  Crimp is also important.  A good medium to firm crimp results in the best groups.  It's also very important that bullets don't pull out a bit and lengthen as as may prevent the cylinder from advancing. You will need to experiment with this to see how it works with you die set.  Too lose and the bullets pull and groups are bad, too tight and groups are bad, just right and the groups get tighter.

I wanted to highlight what worked the best and won’t go into the thousands of bullets that didn’t work quite so well.  Accuracy with Magnum powder testing is ongoing and the results aren’t all that impressive so far.  Gas check didn’t improve or even duplicate the best results, so these are all plain base cast bullets.

The bottom line: soft and fat bullets loaded to fit the throat (sometimes long) over lighter loads are very accurate.  Bullet designs that don’t fit the throat well just aren’t as accurate as those that do.

Sorry to go so long, I tried to keep it short… ttyl

P.S.  Best accuracy for TL bullets is with LLA (used on all TL test bullets) with 2 medium coats followed with motor mica.  This is different than the light coating, or mixing the LLA with mineral spirits or Johnson Paste Wax that others seem to have good results with.

P.P.S.  Sizing to throat size and/or .001 to .002 over groove are not very accurate in my revolver.  These suggested sizings can be found in various places and didn't work.  Water dropping them didn't work either.  My revolver's throats slug at .358 and it has a .357 groove.

P.P.P.S. My rest is a rolled up carpet (taken when my wife wasn't looking) with some tape around each end.  Achieving decent groups off-hand will take many more thousands of boolits.

No comments:

Post a Comment