Duplexing affect pressures ?

[7mmmag]

I was wanting to know how much or less pressure would be if I shot 77 gr 4198 as my load , then duplex 8 gr red dot 69 gr gr 4198 . I am shooting 300 gr bullets in a centerfire action rifle my velocity was 2908 ave with the strait 77 gr load . The priming system takes mag rifle primers . I have a friend with load program but couldn't figure how to put in duplex .

 I shot the rifle fouled the barrel , loaded it at 20 deg let it set outside  the next day in the blind temps warmed and I had sweat all over temps went to 50 . I had misfire so I did unload but smelled like it tried to burn but no ignition , it was not wet as far as I could tell . The rifle had shot perfect at 40 deg ! I have read post on this similair subject but cant find them and I also looked in database for similar loads but did'nt see any .

I think I will start duplexing to help with this problem . I did try 1 shot with the 8 gr red dot 69 gr 4198 after getting home with every thing still cold and it did shoot . any one shooting loads similar ? Thanks 

[vpimine]

Typically, for the fast shotgun and piston powder for every grain of booster you drop two grains of main charge. Therefore if you were shooting 77 g 4198 and add 8 grains of booster ( red dot or clays ) then your primary charge would be 61 grains. Richard shoots a lot of the 10 g clays and 60 g 4198. It is a good load for him in his gun. I only used 5 g clays with my full forms bullets I just one positive ignition in the cold weather. Worked for me every time I pulled the trigger last year in 0 degree temps.

[Richard]

As vpimine indicated, five gr. of Clays/Red Dot will work fine with full formed bullets due to the increased resistance that comes with that set up. With smooths sized bullet there tends to be less resistance therefor I have found a hotter (ten gr.) booster charge get the smooth bullet fattened up and grabbing the rifling. 10/60 Clays/H-4198 with 290/300 gr. bullets
works for me.
Richard

[7mmmag]

Ok thanks , I use smooth sized bullets  in Pac nor barrel . Will the velocity be close to the same for my ballistic chart i already worked up ? 

[vpimine]

In my gun it was close but I would recommend trying in your to confirm.

[ronnie]

So how much clays would go good with 200 grain sst and smooth blue sabot? I now shoot 57 grains h4198 and would like to stay close to this pressure level.

[rossman40]

What you are doing with duplex is changing the pressure "curve". With a faster booster your getting a sharper rise at igntion but then the main charge burn rate will be different because of the higher intial pressure. Smokeless powder burn rate thrives on pressure and trying to calculate the pressure is pretty tough and I wouldn't trust Quickload to come up with a answer even if they started to come up with a way to input it. Just for example BH209 is a powder that acts real squirrly when the pressures get high. Also if one would would use a high energy double base main it could get dangerous. The recent big improvements in powders has been how the retardents (additives to slow burn rate) work. That is why all the "latest and greatest" powders have been slower powders.

What saves us is the work of TG and others to run a pressure trace on new loads. As long as you start with a tested combo you can juggle a bit and still be pretty safe. The faster pistol powders are a lot of bang in a small amount compared to say 4759 as a booster.

[Dave W]

Nov 22, 2014 12:50:59 GMT -5 rossman40 said:

What you are doing with duplex is changing the pressure "curve". With a faster booster your getting a sharper rise at igntion but then the main charge burn rate will be different because of the higher intial pressure. Smokeless powder burn rate thrives on pressure and trying to calculate the pressure is pretty tough and I wouldn't trust Quickload to come up with a answer even if they started to come up with a way to input it. Just for example BH209 is a powder that acts real squirrly when the pressures get high. Also if one would would use a high energy double base main it could get dangerous. The recent big improvements in powders has been how the retardents (additives to slow burn rate) work. That is why all the "latest and greatest" powders have been slower powders.

What saves us is the work of TG and others to run a pressure trace on new loads. As long as you start with a tested combo you can juggle a bit and still be pretty safe. The faster pistol powders are a lot of bang in a small amount compared to say 4759 as a booster.

The Red Dot will almost assuredly increase the pressure above your current load and whatever the estimate on it is.
Like Ken said, the booster spikes the curve and shortens it.

[edge]

Not to disagree, but if you get occasional misfires then the Booster probably only gets you to where it might be in a CF.
The misfire is caused by a lack of pressure.

You definitely only want enough booster to get the main charge up to the needed pressure so a little goes a long way.

Faster powder is not always what you want, some fast powders are not easy to ignite.
Cowboy powders are good because they are designed for fairly empty cases so will go bang every time.

Without a PT I would suggest a chronograph and not shoot faster than the straight powder load and just enough booster for consistency.

edge.

[vpimine]

Ronnie

Bases on Richards testing clays and red dot do not make good boosters for sabot loads. They over spike the pressure and damage the sabot. You will need something like 4759 or other. Someone that sabots may recommend something else. I only shoot smooth and full firm bullets.

[ronnie]

Thanks guys for the help have not had much testing time just reading and learning. Have not had a misfire with straight h4198 as yet bullberry barrel is tight all the way down maybe this helps.

[Richard]

Ditto on vpimine's post. Clays as a booster with sabots is not what you want. I has blown sabots every time I tried it.......even in small increments. Some 4759 or N-110 are better boosters for saboted shooting.
Richard

[7mmmag]

Thank You gentleman , I got to shoot once before the rain , I will probally drop charge to 5 gr RD 63 gr 4198 . The rifle  shot excellent with 40 deg. temps and mabey condensation , humid  weather had more to do than I thought .