Range Report: Barnes XPB [Mission Aborted]

[pitted bore]

Part of my concern with the Barnes 200's is they are so short and .4505 diameter. Consider the free bore that the 450 AR's have (0.10"). I may be wrong, but I don't think BD's bolt gun has the free bore and also he is running a .451 inch barrel. Is this correct? So -- with the AR, the bullet is nearly out of the case just as the center driving band is barely engaging the rifling. IF the case is not perfectly square to the bore -- it would be easy for the gas pressure to cock the bullet to one side or another,since the free bore is around .453" diameter -- plenty of room for a .4505 bullet to bounce around if not securely in the bore. I think this is a problem with most of the short bullets. There are a lot of factors against getting these to shoot well in the Bushmaster barrel.

Siringo-
This post opens a new line of thinking. Thanks (I think). I hope the following comment does not derail the thread.

First, the blanks of the barrels on both my bolt rifles were stated to conform to nominal .45 auto dimensions. However, these dimensions as set by SAAMI have really broad limits. Bore can be 0.442" to .446"'; groove from 0.450" to 0.454". I have not slugged either barrel to find their actual dimensions. I have not seen the SAAMi specs on the 450B barrel for comparison.

(The jacketed bullet diameter for the 45 auto should be between 0.4490" to 0.4520"; for the 450B the Hornady drawing shows 0.4495" to 0.4525".)

Directly addressing the question about freebore in the bolt guns: because the chamber reamer for my guns was ground to 450B specs, the chamber, throat, leade, and any freebore characteristics should be the same as for the ARs.

Because I've been concentrating on velocity, I have not measured bullet-land engagement distances for any of the three bullets I've been using. I'll put it on my list of things to do.
--Bob

[Siringo]

Thanks for the correction. I was confused who had what!? Too late last night and in the heat all day.

[BD1]

I'm not going to worry too much about whether any of the bullet shank is in the case when the ogive gets to the lands. Unless you've got a custom bench rest chamber reamed into your barrel with your brass turned to exactly match it, minus bullet diameter, the case isn't in contact with the bullet at all when the ogive gets to the lands. If you are using sufficient crimp that brass should be slapped out tight to the chamber walls with about .001 clearance all around by the time the 40,000 psi cloud of gas is forcing the bullet past the throat into the lands. Cartridge concentricity is certainly important so that the bullet has the best alignment possible to the bore at the instant the big bang occurs, but IMHO unless you're running high SD bullets over slow powder in a match chamber with fitted brass the idea that the case is somehow "guiding" the bullet at ignition is wishful thinking. Particularly in something like the AR platform where we need to purposely leave some clearance between the case and the chamber to insure proper feeding.
BD

[pitted bore]

Siringo-
My post was not intended to be a correction. It's sort of flattering to be confused with BD1, a suave and debonair connoisseur of fine firearms and arcane reloading techniques <grin>.

I have been ignoring the problem of tolerances in bore and bullet dimensions, except as the latter made loading difficult. As soon as I get over my current need-for-speed phase, I'll have to start paying attention. Hopefully the forum will have it all worked out so I won't have to do a lot of trial-and-error then.

--Bob

[BD1]

"It's sort of flattering to be confused with BD1, a suave and debonair connoisseur of fine firearms and arcane reloading techniques <grin>. "

Obviously he's never met me in the flesh, <burp>

[wildcatter]

Hoot, about your case-head expansion. Did you do a before shooting measurement, to compare to the after? I always do and I number the cases, so that the measurements, are before and after, the ones I actually measured, concurrently. Without the before/after, one will never know if they are in trouble. Of course different lots will expand at different rates/amounts and case head expansion is just one of many pressure indicators..

..t

[Hoot]

Hoot, about your case-head expansion. Did you do a before shooting measurement, to compare to the after? I always do and I number the cases, so that the measurements, are before and after, the ones I actually measured, concurrently. Without the before/after, one will never know if they are in trouble. Of course different lots will expand at different rates/amounts and case head expansion is just one of many pressure indicators..

..t

snip...Just a footnote worth mentioning. My resized brass typically mics around .501 to start with. That how my dies came and they still have plenty of wiggle room in the Bushmaster chamber, so NBD...snip

The 35 unfired rounds averaged .501 just now. Not a lot of variation. Like .5008-.5011 according to the mic.

Hoot

[wildcatter]

Hoot, about your case-head expansion. Did you do a before shooting measurement, to compare to the after? I always do and I number the cases, so that the measurements, are before and after, the ones I actually measured, concurrently. Without the before/after, one will never know if they are in trouble. Of course different lots will expand at different rates/amounts and case head expansion is just one of many pressure indicators..

..t

snip...Just a footnote worth mentioning. My resized brass typically mics around .501 to start with. That how my dies came and they still have plenty of wiggle room in the Bushmaster chamber, so NBD...snip

The 35 unfired rounds averaged .501 just now. Not a lot of variation. Like .5008-.5011 according to the mic.

Hoot

Shore-Nuff, but did you mic them before firing, to be able to ascertain the total case head expansion?..

..t

[Hoot]

The 35 unfired rounds averaged .501 just now. Not a lot of variation. Like .5008-.5011 according to the mic.
Hoot

Shore-Nuff, but did you mic them before firing, to be able to ascertain the total case head expansion?..

..t

As the quote said the unfired ones averaged .501. All my resized cases that make the unfired cartridges I take to the range are .501.

Applying the math:
Fired Brass 38gr = .503
Unfired = .501
Expansion = .002
Likewise, 39gr = .504
Unfired =.501
Expansion = .003

Given the chamber wall imprinting on the .39gr loads, I doubt I would have gotten more than .002 more expansion until some part gave out like the lugs or worse, as I advanced up through the stronger charges.

Hoot

[wildcatter]

Oops, I'm new at this read'en thing..

Yup, three thou is allot, but livable, imho. I can live with three, but I prefer less than two..

..t