best GtoB ?

[mtngun]

The next TC 357 rifle shootout:

Different GtoB's on an otherwise identical 160 gr. plain base spitzer.

In the event the 1.8 GtoB shoots better than the 0.6, people might assume "it's because the 1.8 holds more lube." To test that theory, we'll include a 1.8 that only has one groove lubed, so that it actually carries less lube than the 0.6.

Which do you think will shoot best?

[mtngun]

Unless otherwise stated, all loads today used my usual load of 17.9 gr. WW 296, a Winchester small pistol magnum primer, oven treated wheelweight, sized 0.360", HVR lube, and 1.840" COL (tho the COL varied a bit , more on that later). 27" Green Mountain barrel with a sloppy throat on a TC action.

Because the GtoB changes the length of the bullet slightly, that in turn changes how far the bullet seats into the case, which in turn changes the velocity. So we expect the velocity may be different for different GtoB's.

Temperature was a nippy 15 - 20 degrees outside, with a 5 - 20 mph breeze in my face. Temperature inside the shooting shack was 50-ish.

Procedure was to shoot three 10-shot groups for each load. The first group was for seasoning and warm up, and is not counted because it is common for a cold barrel to require several warm-up shots before it settles down. Only the 2nd and 3rd group results were counted in the final summary An exception was made for one load, more on that later.

First up, the "old reliable" GtoB 1.0 bullet that I have tested several times before. I tested it again because I believe in always including a "standard" load to compare against experimental loads.

Note that the 1st shot from the cold barrel was a good 12" to the right. If I had been shooting at a deer, it might have missed or wounded the deer. Makes you wonder.

-- 2012 fps velocity
-- 1.32% standard deviation (poor, and not up to this load's usual 0.94% - 1.07%)
-- 0.75" mean radius (not up to this load's usual 0.54" - 0.55")
-- 2.15" MOA


For some reason this "old reliable" load was not shooting as well as it has in the past.

Also, twice the action popped open when this load was fired. The first time it happened, in group #2, it resulted in a flier which I did not count (it's marked "called flyer" on the target). The 2nd time it landed within the group.

It's possible that I was careless in closing the action, or it is possible that the COL was too long and making it difficult to close the action all the way. If so, that could explain the sub-par accuracy and the sub-par velocity standard deviation. I know from experience that TC's do better with the bullet seated so that the action closes easily, even if it means that the bullet has to jump to the rifling.

I had spot checked the COL of a couple of rounds and they were running a few thou over the 1.840" target. I didn't worry about it at the time, but maybe I should have.

On to the next bullet .....

[mtngun]

Next up, the 0.6 GtoB bullet. The barrel was already warm when I started shooting this load so the seasoning group was just as good as the next two groups. Nonetheless I only counted the 2nd and 3rd group since that was the plan.

-- 2009 fps (the 0.6 GtoB is shorter than the others, so it doesn't seat as deep, so the velocity is a bit lower)
-- 1.04% velocity standard deviation, decent.
-- 0.693" mean radius
-- 2.48" MOA
-- there was no fouling visible to the naked eye at the end of this string.


In case you are wondering, the numbers on the bullet holes are simply to keep track of which holes have been measured and do not refer to the chronological order of shooting.

On to the next bullet ....

[mtngun]

Next up, the 1.8 GtoB bullet with all grooves lubed. This time I was more careful with the COL, and turned the seating stem in half a turn so that the COL was 1.836" - 1.840".

Also, it so happened that the case mouth ended up in the middle of a lube groove, so that I could crimp it harder than the other bullets.

The barrel had cooled back down while I was loading these, so the first shot was wild, then it settled down. Only the 2nd and 3rd groups were counted as per the plan.

-- 2030 fps (it's longer and seated deeper than the other two bullets, hence the higher velocity)
-- 0.64% fps velocity standard deviation, very very good.
-- 0.76" mean radius, ho hum
-- 2.65" MOA, ho hum
-- of course there was no visible fouling at the end of this string


Next up, the 1.8 GtoB bullet with only one groove lubed. The barrel was already warm so the seasoning group was no worse than the 2nd and 3rd groups.

For the 2nd group, I deliberately tried a different benchrest technique, just lightly touching the butt of the rifle with my shoulder instead of pulling the butt snug against my shoulder like I usually do. That seemed to result in vertical stringing so I threw out that group and counted 1st and 3rd groups, instead.

-- 2034 fps
-- 0.82% fps velocity variation, higher than the fully lubed bullet, but still lower than the other two bullets. Hmmm.
-- 1.09" mean radius, poor
-- 4.0 MOA, sucks.
-- there was no visible fouling at the end of this string, though the barrel wasn't quite as shiny as the other bullets, if that makes sense.

Conclusions and lessons learned next .....

[mtngun]

Mean radius. Only the partially lubed bullet was statistically different from the rest, as we'll see later.


Standard deviation of velocity for the 20 shots. The 1.8 GtoB was definitely superior in this category, more on that later.


Results of student's t-test, based on the distance from center for 20 shots:
-- 5.84% probability that GtoB 1.8 was different than GtoB 1.0. In other words, no significant difference.
-- 91.68% probability that lubing only one groove was worse than lubing all grooves
-- no statistically significant difference between the 0.6 GtoB and the other two bullets.


Why Did My "Old Reliable" Load Not Do As Well As Normal?
-- it could have been the cold weather, though it wasn't THAT cold.
-- but I suspect the COL was longer than optimal. TC's can be very sensitive to COL.

Why Was The Velocity Standard Deviation So Much Better on the 1.8 GtoB?
-- I had decreased the COL a wee bit, so that it chambered easier.
-- the longer bullet seated a wee bit deeper into the powder space.
-- I was able to crimp it harder than the other bullets, because of the way the grooves lined up with the case mouth.

Conclusions and Lessons Learned:
-- GtoB doesn't seem to be all that important, at least not in this load, in this rifle.
-- but it is important to fill all the grooves with lube, at least in this load, in this rifle.
-- if I were going to keep shooting this barrel, I should try again to optimize the COL.
-- if I were going to keep shooting this barrel, I should make a bullet that can be crimped hard.
-- you wouldn't think a 13 1/2 pound rifle with a bull barrel would be tricky to shoot off a benchrest, but this TC is. Perhaps because of the break open action and 2-piece stock, the point of impact is surprisingly sensitive to benchrest technique. I have no solution other than practice, practice, practice.

Things to Try Next Time:
-- there may not be a next time for this barrel because I'm ready to address its sloppy throat. I haven't decided whether to punch out the chamber to 357 Maxi, or to set the barrel back and rechamber to 357 mag. Or eventually try both?

[jbquack1]

Looking at the bullets, my guess would have been thje 1.0 G to B. It just looks uniform, but as you said, it doesn't seem to matter much.