Friday, June 22, 2007

It's not the rifle.

Readers will recall that in this post I discussed what happens when one changes handloaded components. In that post I noticed that a 168gr Sierra HPBT bullet, atop 42.5 grs of 4064 powder, in a BHA case would group with deadly accurate consistencies, whereas an identical load but built with FC match cases wouldn't group at all. Since the latter produced velocities about 5% faster than the former, I was left to speculate whether the discrepancy was due to

  1. the brass design of BHA brass (more consistent neck tension? more consistent bullet release?), or
  2. the rifle had a sweet spot for 168gr HPBT bullets at around the 2550fps mark.
So, I worked up a set of loads using only FC match cases, while varying the powder weight from 39.6grs to 42.5grs with a 0.5gr delta. Here are the velocity curves, as compared to previous measurements with BHA brass.

As one will recall, the load built with BHA brass produced an accuracy (and velocity) sweet spot at around 42.5grs of 4064, which also corresponds to the inflection point on the powder weight vs. velocity curve -- YEEHAW, theory matches empirical measurements.

Sure enough, an inflection point was observed with FC brass also, at 41.0 grs and a velocity of 2529+-15.3fps. But the accuracy at this point wasn't stellar; sub-MOA, for sure, but not 0.3MOA as was observed with BHA brass.

Now, in the 2550fps region, there was a velocity consistency sweet spot at 41.5grs of 4064, for a velocity of 2575+-10.8fps, but accuracy here was even worse. Sure, there's 3 shots through the same hole, but two seriously bad fliers.

The best accuracy was achieved with 40.0grs of 4064 for a velocity of 2444+-21.5fps, but even here there's one bad flier.

Ironically, the best velocity consistency was achieved at 42.5grs of 4064 for a velocity of 2658+- 5.1, but accuracy here was very poor.

So what does this all mean? Theory still predicts practice, in that an inflection point in the powder vs. velocity curve does imply a sweet spot from a consistent accuracy standpoint. But, at least in this case, trying to achieve sub-0.5-MOA accuracy with FC match brass is an exercise in turd-polishing. It ain't gonna happen. Though my rifle does like 168gr bullets to be travelling in the 2550fps region, there's something about BHA brass that really works in my Remington 700PSS. Good thing I recently bought 200 more rounds of BHA brass.

It just occurred to me that FC brass might have thicker necks than BHA brass, and since I'm using the same neck-only sizing dies, the resultant loads built with FC brass may have greater neck tension than the BHA brass. Hmmmm ..... perhaps there's hope yet for FC brass, if I spring for Redding competition dies with changeable neck inserts. More experimentation is required, perhaps with Winchester brass also.

And the verdict is ....

Measured a couple of case necks, going around, 90degrees at a time, and measuring.

BHA -- 0.010"-0.011"
FC -- 0.012"-0.014"

  1. BHA cases have thinner necks than FC match cases. Since I use the same neck-only sizing dies, this will result in greater neck tension in the case of FC and hence greater release inconsistency and potentially more damage to bullet jackets.
  2. BHA cases have more consistent neck thickness, resulting in better bullet concentricity with the bore.

There ya have it .......


Weetabix said...

This may be a newbie type question, but please be gentle.

You get those great groups. Do you shoot this from a rest or do you have some contraption that locks the rifle down so you know the POA is exactly the same each time?

I guess I'm asking, "How do you shoot so good?"

Felix Estrella said...

No, no contraptions. Just sandbags and, fortunately, good eyes.

Weetabix said...

FELIX! Felix! felix....

(my version of an echo on the internet)

Please write! Your blog is in the top 3 or so I look to each day. I'm having withdrawal symptoms. :-)

Felix Estrella said...

SIR, YES SIR!!!! Loud and clear. :-) I have a posted "primed" about high pressure 45 ACP defensive load workup.