169.5 RBBT in .270-300 Win Mag Help wanted

[Stevewix]

I started my first round of handload development for the 169.5 ULD RBBT from Richard (Wildcat Bullets) yesterday and ran into a snag. First shot with 72 gr. Retumbo, OAL 3.51" (bullet .017" off the rifling) netted me a cratered primer but no extractor mark OR heavy bolt lift. I thought nothing of it and fired the second shot, garnering myself the heaviest bolt I've ever lifted and a primer that fell out on it's own. The remaining 23, each 5 in .5gr increments were disassembled.

This is the first load I've done in this particular gun with the bullet close to the rifling. I wish to use this heavy bullet/high bc as a long-range hunting/match shooting combination, so accuracy is the first and foremost intent.

The other loads I've successfully started low and worked my way up on, are as follows.

Varmint / Close Range Target:
110 Gr. HDY V-Max, 79 gr. RL22, CCI 250LRM primers, 2.61" case length, 3.23" OAL, 3809 fps from 28" tube (5 shot avg). Regular sub .400" 5 shot groups at 100 yards.

Hunting:

140 Gr. Nosler Accubond, 71.5 gr RL22, CCI 250 LRM primers, 2.61" Case length, 3.35" OAL, .75" 5 shot group at 100 yards, 40mph sidewind.

My case capacity is 92.4 grains (fired).

Should I be using a slower powder? Suggestions?

Also, do the Wildcat 169.5 RBBTs require more engraving pressure? If so is it because of the long bearing surface?


Any help is appreciated.

Steve

 

[Stevewix]

I keep going back to the drawing board and cannot find anything wrong.

Has anyone ever seen pressure spike through the roof merely by seating the bullet closer to the lands?

I'm tempted to try going down 6 grains and starting again.

 

[brianwinzor]

I havent used a 27/300WM, but I am using a 257Weatherby (no freebore) with the Wildcat 156 grain ULD.

The capacity of my neck sized Norma 257W cases is 90.8 grains, so it is similar to the 92.4 grains for your fired case. When I form cases from 300 WM brass the capacity is about 2.5 grains less. (88.3)

I got good accuracy with the bullet seated .015 in from the lands, so I believe your seating depth of .017 in would be a good place to start.

From personal experience, a blown primer usually means about 3.0 grain overload depending on the type of powder and temperature. After a little research, I believe that starting at about 64.0/Retumbo with the 169WC, and working up to maximum pressure from there should put you in a safe pressure zone.

Personally, I would switch to a slower powder to achieve a better loading density, as if a safe maximum load of HRetumbo turns out to be in 67 - 69 grain range, then you probably have a load density of about 81% depending on how your barrel is throated. I prefer to use a powder (if available) that will achieve at least a 95% load density at maximum pressure, and even better close as possible to 100%.

In my 257W with the 156 grain WC, I reached maximum pressure with 65.0 grains of AR2225 (HRetumbo), and 70.0 grains AR2218 (H50BMG). Good luck on your project, and hope the above is of some help. Brian.

 

[Stevewix]

I havent used a 27/300WM, but I am using a 257Weatherby (no freebore) with the Wildcat 156 grain ULD.

The capacity of my neck sized Norma 257W cases is 90.8 grains, so it is similar to the 92.4 grains for your fired case. When I form cases from 300 WM brass the capacity is about 2.5 grains less. (88.3)

I got good accuracy with the bullet seated .015 in from the lands, so I believe your seating depth of .017 in would be a good place to start.

From personal experience, a blown primer usually means about 3.0 grain overload depending on the type of powder and temperature. After a little research, I believe that starting at about 64.0/Retumbo with the 169WC, and working up to maximum pressure from there should put you in a safe pressure zone.

Personally, I would switch to a slower powder to achieve a better loading density, as if a safe maximum load of HRetumbo turns out to be in 67 - 69 grain range, then you probably have a load density of about 81% depending on how your barrel is throated. I prefer to use a powder (if available) that will achieve at least a 95% load density at maximum pressure, and even better close as possible to 100%.

In my 257W with the 156 grain WC, I reached maximum pressure with 65.0 grains of AR2225 (HRetumbo), and 70.0 grains AR2218 (H50BMG). Good luck on your project, and hope the above is of some help. Brian.

Thanks so much, H50BMG the way to go? Will I be able to dispense that stuff from a powder drop or will it bridge? Thoughts on US869?

I've got a .257 Wby also, what kind of velocity/accuracy do you see with the 156's?

 

[royinidaho]

Stevewix,

Beware of powder bridging with those stick powders.

The only reference I have is my experience with my 270 AM. I was doing some messing around with 150 class bullets and got inexplicably high pressures with Retumbo, RL-22 etc.

After some discussion with Kirby Allen the cause was determined to be bridging. Bridging is where the powder actually plugs the case at the base of the neck for a period of time.

You may wish to go to the ball powders. The 270 169 RBBT was designed for a cartridge with large capacity and sharp shoulders which condones bridging. I wouldn't consider using the 169 w/stick powders in your case.

Use US-869 and I bet your problems will go away.

 

[brianwinzor]

SteveWix, I havent recently tried dispensing H50BMG and other stick powders through a powder thrower. For safety reasons, I weigh out each charge when testing, and/or for my near maximum/maximum field loads.

I have fired over 250 rounds in my 257W using H50BMG and there has never been a hint of powder bridging, so I will continue to use it. However, I may have experienced powder bridging with Viht 20N29 which has slightly longer and thinner powder granules.

Using US 869 as RoyinIdaho suggests will certainly eliminate the powder bridging issue, and as you are trying to get stable and predictable pressures it probably is the best option for you at this stage.

I haven't tried US-869, as I don't believe it is available in Oz. However, I have tried H870 in several different cases, and found that (like most ball powders) it is much more temperature sensitive than H50BMG.

My 257W (7 twist Krieger 26 inch barrel) produces 2,900 fps with the 156WC when using Norma 257W brass, and about 2970 fps when using 300WM brass. I have had no trouble in getting consistent sub 0.5 moa accuracy with the 156, and it certainly cuts through the wind better than any other bullet I have used so far. Brian.

 

[Fiftydriver]

If I am reading your information correctly, your using 71.5 gr of R22 with the 140 gr Accubond and started with 72.0 gr Retumbo under the 169.5 gr ULD RBBT?

Retumbo is slower then R22 but not dramatically slower, I would say start at around 65.0 gr and work up from there.

Another tip, when you start a new load work up with a bullet you are not familiar with, back the bullet at least 50 thou off the lands just to give you a bit of a pressure cushion in case you start out a bit to hot as you did in this case.

Looking at your round, I would say Retumbo will work well but you may also want to try some ball powders. If you have access to Ramshot Magnum powder, I think you will be pleasantly suprised at this powder in your wildcat.

AA8700, US869 and WC872 should work fine but I doubt you will be able to get anywhere near top working pressures and because of that these powders will burn very dirty. Ramshot Magnum has a burn rate similiar to Retumbo, a bit quicker and much more appropriate for your case capacity.

Once you get a feel for the personality of your wildcats bullet/load combo, then you can work the bullet up to the lands when you know its safe to do so.

Kirby Allen(50)

 

[Stevewix]

Thanks Fifty, I was always under the impression Retumbo was drastically slower than RL22. Insight appreciated!

 

[Stevewix]

Edited.