How did I get an extra 150fps? 300Wby

[TXFlyer]

This may be a stupid question, but on the surface it's perplexing for a simpleton like me.
Here's the data, an ideas what may be causing this?

300 Weatherby Accumark, same gun, same brass/primers, same powder load, same OAL:

168 gr Berger Classic Hunter (BC .496) = 3125fps

168 gr Nosler BT (BC .490) = 3278fps

I have a theory but tell me what you think.

 

[BrentM]

Bearing surface or seating depth of ogive to the lands? Is it just one test or multiple tests with similar results? Crazy amount of speed increase.

 

[TXFlyer]

It's a Weatherby, so not sure about distance to lands (freebore). I set the OAL as long as possible. This was the first test of the loads--5 rounds each and I averaged the velocities. The faster group was shot after the slower group but I let the gun cool several minutes.

I'm guessing it's probably bearing surface but haven't looked closely at bullet dimensions yet.

For what it's worth, the Berger group was .666, the Nosler group 1.00.

 

[BrentM]

I think you have an increase in pressure on the nosler group due to the design of the bullet putting the ogive closer the lands. Bearing surface will have some affect but for example on my 6.5 .100 from the lands with the same powder vs .000 to the lands produced 130 fps difference, with the same bullet. The pressure was off the chart though and looked like was about to blow holes in my primers.

 

[TXFlyer]

Lengths:
Berger 1.254"
Nosler 1.304"

I think I might be getting more case pressure because of the length and shape of the Nosler (seated deeper because it's longer). The Berger looks like it has less bearing surface, so I don't think that's the culprit.

Any suggestions for seating depths on a Weatherby Accumark with a freebore barrel??

 

[BrentM]

sorry no. I always measure the ogive seating depth via a Hornady OAL guage or the old method of inserting a bullet in a case and chambering it. I always record the data for each bullet design and set my dies to achieve the OAL to the OGIVE that produces the best accuracey etc. I adjust powder accordingly as well.

Someone on here should be able to help. I am no expert so my help is limited.

 

[Bart B]

If your barrel's bore and groove diameters were smaller than what was used to get the load data and you held the rifle differently, too, that extra muzzle velocity is normal. The use of different primers and powder lots as well as firing pin impact force, case neck tension than what the load reference used compounds the issue.

It's not abnormal to shoot a given load's bullet 200 fps faster or slower than what some published load says. All sorts of variables that are not listed for comparisons.

 

[BrentM]

OK, so you are saying that you can get 150 fps difference by doing nothing more than changing bullet manufacture? I mean, the OP did say that is the only thing that changed.

I learn something new everyday. I am going to test the 140 SMK, 140 Amax, etc to see if I can get 150 fps more from my 6.5 then what my 140 vlds are doing, that would be fantastic.

 

[backwoods83]

Yes you can gain or lose velocity by switching nothing but bullets. I have went from 140amaxs to 140 Bergers, both seated to the lands and had to add .6grns to the Berger load to match the velocity, doesn't bother me though, either shoots under 1/4moa. In the OPs case the NBT is longer, has a longer bearing surface and a thicker jacket than the Berger, these all aid in adding extra pressure or in turn extra velocity. Btw 32xxfps with a 168 is very tame, that's on 24" barrel 300wm level. The Bergers or Noslers will both safely go over 3400 with H4831 or Imr7828.

 

[Shodan]

I think that part of the issue is that the Berger bullets are a secant style, and the accubonds are a tangent. It plays with the distance that the different bullets styles have to jump to the lands. I shoot the Hornady A-max and Nosler Accubonds out of my 7mm (a remington), and have matched their velocities appropriately to have their trajectory's match. I had to play around a bit with the OAL as well as the powder charge for getting the velocities that I wanted, and I ended up with a smaller powder charge and shorter OAL on the Accubond load, and a longer, larger charged load with the a-max. I did load some rounds with the Accubond that were the same length and powder charge as the A max, and after firing one had to put them to the side and pull them apart when I got home. Way too hot. It was that the bullet was too close to the lands with too large of a charge. I haven't loaded for a weatherby, but i would just start with the standard OAL and charge, and then find what seating depth works best for accuracy for each bullet. I would then up the powder charge for each load until you get the velocity that it should be going at. You might have a smaller charge, but you are running at the same pressure because of the bullet being closer to the lands, and jumping the pressure up sooner.

I wouldn't recommend changing bullets in a given load from a secant style to a tangent style just because of the tangent bullet contacting the lands sooner, creating higher pressures, and thereby more velocity. The problem is that you would be over pressure. A lot of rifles might not even show "signs" of high pressure. If it is made very well, and true to itself, you won't experience a sticky bolt. If you have a tighter chamber, you might have zero brass flowing. But you would still be over pressure. Most rifles can handle it just fine, since they are proofed to much higher levels, but it is still risky. What if you're using that too hot load, on a warm day, and are sitting in the sun with your rifle out? That might turn out some problems if you shoot.

 

[backwoods83]

/\ the shape of the ogive means nothing pressure wise, in a 300 wby the bullet starts engraving at .300" no matter where or what shape it is when the bullet comes to that diameter. Also being closer to the lands with 2 identical bullets is not going to show more pressure with the same powder charge because that leaves more air space in the case and no more resistance on the bullet. Now if said bullet was jammed in the lands then yes it could be over pressured without reducing the charge weight due to a 4500 psi higher start pressure. You mention the hornady verses the accubond, since the accubond has about .125" longer bearing surface (162 amax vs 160 accubond) it creates much more pressure because of the amount of surface area that is being engraved by the rifling.

 

[Bart B]

/\ the shape of the ogive means nothing pressure wise, in a 300 wby the bullet starts engraving at .300" no matter where or what shape it is when the bullet comes to that diameter. Also being closer to the lands with 2 identical bullets is not going to show more pressure with the same powder charge because that leaves more air space in the case and no more resistance on the bullet.

I think there will be different peak pressures for different amounts of bullet jump to the lands.

And as 30 caliber bullets' diameters range from about 3070" to over .3090" and their ogive shape diameters at any place in that range will contact the rifling anywhere in it depending on the chamber's throat diameter and leade angle on the rifling from that diameter past groove diameter (which is often smaller than the bullet's diameter) down to bore diameter. And bullets with a longer bearing surface will have a longer time of being engraved by the rifling and that'll cause different pressures with them.

 

[backwoods83]

Bart I know your sole purpose in life is to prove others wrong, but last I checked a standard 30 cal barrel was .300 on the lands and .308" on the grooves, since the lands start the engraving how does this occur between .307-.309", do tell oh wise one!

 

[TOM H]

Lengths:
Berger 1.254"
Nosler 1.304"

I think I might be getting more case pressure because of the length and shape of the Nosler (seated deeper because it's longer). The Berger looks like it has less bearing surface, so I don't think that's the culprit.

Any suggestions for seating depths on a Weatherby Accumark with a freebore barrel??

I would run Bergers over a chrono again see what you get. I had 300WSM build and I work up loads using TSX 165gr/TSX 168gr,E-Tip 168gr and 165 gr Accubond and my seating depth was for the TSX's and I use that for the 165gr AB and 168gr E-Tip.

66gr/R-17-- 165gr TSX @ 3208fps,168gr TSX @ 3226fps,168gr E-Tip @ 3159fps and 165gr AB @ 3176fps and high to low velocity spread was 67fps. I was going to change the seating depth on the AB,E-Tip but ran out of daylight.

Berger recommend 1/13 twist barrel for the 168gr bullet you were shooting and I think than may be one problem.

Well good luck