Recently loaded and shot a 43 Mauser with six loads between 14gr. and 17gr. of Trail Boss powder with 15.75gr. being the 70% mark. Bullet was .446cal 368gr. lead cast; primer was Winchester large rifle. All loads showed DANGEROUS PRESSURE signs and almost had to beat the bolt open.

(No safety flames here, after the first shot was was happening was realized and continued firing from behind a barrier out of a rest for safety). Unfortuntly did not bring the chronograph to the range.

The same Mauser rifle was shot with the same components using 29gr. of IMR 4198 and this load was gentle with no extraction issues. This time the brass showed no pressure signs at all.

Also shot a .43 Spanish using 14gr. to 17gr. of the same lot of Trail Boss using the same Winchester large rifle primers and .439 385gr. lead cast bullets. The 70% mark is 15.5gr. in this brass. All of these loads are also gentle, with no extraction issues / no pressure signs at all.

The 43 Mauser and 43 Spanish placed side by side are very similar cartridges. I have always read and been told that Trail Boss is designed to not produce high pressure. Have used a lot of it and never seen this happen before. Does anyone have any input on what is going on?

One of the fired 43 Mauser cases is being mailed back to IMR with a letter asking for their analysis. I will update this thread once I hear back.

Are you, by chance, COMPRESSING the Trail Boss load?

I use 6.2 grains in a 44 Magnum and yet don't think that you would get with 17.0 grains, in a .43 Mauser, a compressed load?

Certainly I have read that when it is compressed it "goes into nuclear meltdown" as one commentator said.

Or are you, somehow shearing or breaking those doughnut shaped kernels. Which I guess would also do the same.

Good thought, and I will recheck all measurements tonight but as far as I know no compression. The pressure signs were with all loads, even those below 70% load density. Only scoops and a trickler used for the powder, nothing automatic.

I did read in another forum a post of by a gentleman using 20.0gr. in his 71/84 Mauser and being very happy with it chronographing at 1400fps to duplicate the military load. The loads I'm questioning were no where near that.

Yes, the .43 mauser and .43 Spanish is smiliar. When a teenager, used to shoot .43 mauser ammo in my 43 Spanish RB and use the fireformed brass for reloading. Actually still have some of this brass that I shoot in my 43 Spanish using Trail Boss powder.

But according to Lee manual, the 43 mauser has a useful case capacity of 3.68 cc and the 43 Spanish is slightly larger, with a useful case capacity of 4.19 cc.

Personally, I'm kinda satisfied using 13 grs of TB with my 370 gr cast .439 bullets. I suggest you reduce your .43 mauser loads to around 13 grs TB and try it. 13 grs should be close to 2.8 cc and 14.3 gr TB is close to 3.1 cc. To me, that is getting close to the Lee specs on the useful case capacity of a 43 Mauser, which Lees says is 3.68 CC.

Also, while I doubt its the problem, confirm the bore diameter of your .43 mauser is .446.

Remeasured Trail Boss in the brass being used: 28gr. to the top of the enpty shell, 22.2gr. to the base of a seated bullet, 15.5gr. = 70% as Hodgdon recommends. Can't swage and measure the bore right now but considering the IMR 4198 loads were fine I don't see how that can be the issue as the same bullets were used.

Interesting. I was at the range this AM shooting my 71 Mauser. I use 20.0 grains of TB with both 340 and 370 gr. bullets without any problems whatsoever. As an aside, I tried some 5744 for the first time today. 25.0grs with both bullet weights worked quite well.

enfieldspares, where did you read about the "nuclear meltdown" ? Hodgdon makes no mention of such danger in it's data for Trail Boss.

12.5gr of TB in my 44 center fire Vetterli ,330 grain cast bullet 1150 fps. no pressure sign and the Swiss cartridge appears to be a lot smaller than yours. roger

Huff,
Where did you get your bullets? I have a 43 Spanish and dies. I'd like to try TB.

Been so long since I bought my .370 gr cast .439 bullets I'm not sure. Do know Midway had some cast bullets for the .43 Spanish, but were paper patch ones. Anyway, may have been Track of the Wolf in Minn., where I got my bullets and some brass too that are reformed from 348 Win brass.

Rusty,

Check with BACO http://www.buffaloarms.com/ or GAD Custom http://www.gadcustomcartridges.com/#cast

I have a new update:

Purchased a new minty rifle and the same loads in it are fine using all powders including Trail Boss. I must assume there is something off with the first rifle's head spacing or chamber that was brought out by the Trail Boss.

Is there any chance that the first rifle's bolt was full of grease causing poor ignition? I had a problem with this using a different powder long ago. I was at the range shooting identical rifles in the same caliber. The other rifle did not have the high pressure problem with the same loads. Finding the grease I swapped bolts and the high pressure went away. Once the guts of the bolt was cleared of the cosmoline. It too functioned normally. I eventually wound up using a load that was 3 grains (39 grains was increased to 42 grains) higher.

I'm weary of trail Boss. I have a 303 Brit with a rust damaged chamber in the neck area. Normal loads are fine but fast powder (shotgun) will lock the neck into the rust recesses without showing any pressure signs anywhere else. Trail Boss is as fast as shotgun powder.