Adjusting OAL for different bullet shape with no load data

[NetworkSkullRipper]

I have been using RMR 124gr FMJ RN bullets along with Alliant Sport Pistol to reload 9mm rounds. I got the load data from Alliant's website and I'm basically using 4.3gr and an OAL of 1.125"(+/- 0.003").

I wanted to try RMR 124gr FMJ TC, but there is not load data for TC vs RN.
In theory, having the same bullet weight and same case capacity I should be able to use the same powder charge safely, right?

I measured the length of the bullets and the RN is 0.592" long while the TC is 0.557" long.
Is it correct (and safe) to shorten my OAL in 0.035" in order to have the same case capacity?

[Cheep]

Unless you are at or near max powder charge, I don't think I would worry about it as long as it cycles in the gun you use.

[tonyjr]

using go / no go al gages - don't count on them . They are for one shape bullet .
I think you know that but others reading this might not .
Shortening the oil means less space for powder . Less space means more pressure .
I / we so it does not matter much .
But , some chambers want the bullet to fully seat , other wise the bullet will hit the side of
chamber and scrape copper or lead off .
i / we do the OAL AND STAY WITH IT . We generally are on low to mid powder charge - so
pressure is not as high as others who go to max powder or close to it .
When I was hunting I stuck with store bought core lock [ for rifles ] and hard ball [ FMJ ]
for the 45 ACP and the 50 AE .
One thing you will find is the less pressure , the easier it is to clean barrels and gas chambers
Plated bullets are worst the jacketed ones . There are going to be posts saying - wrong - but
try and make up your own mind .

[KevinA.]

When I’m changing to a different bullet in the same wight I do as you did and measure them and adjust my oal to maintain the same internal volume. If it still cycles in your gun you should be fine.

[JagerDog]

using go / no go al gages - don't count on them . They are for one shape bullet .
I think you know that but others reading this might not .
Shortening the oil means less space for powder . Less space means more pressure .
I / we so it does not matter much .
But , some chambers want the bullet to fully seat , other wise the bullet will hit the side of
chamber and scrape copper or lead off .
i / we do the OAL AND STAY WITH IT . We generally are on low to mid powder charge - so
pressure is not as high as others who go to max powder or close to it .
When I was hunting I stuck with store bought core lock [ for rifles ] and hard ball [ FMJ ]
for the 45 ACP and the 50 AE .
One thing you will find is the less pressure , the easier it is to clean barrels and gas chambers
Plated bullets are worst the jacketed ones . There are going to be posts saying - wrong - but
try and make up your own mind .

Not if the bullet is that same amount shorter.

What gas chamber?

[tonyjr]

All my semi autos have gas chambers . I only own 3 revolvers .
A shorter bullet does not matter as much in a revolver as in a semi .
plated and coated - more stuff in gas chamber if shorter .
My 22 lr semi's really build up stuff , but they are plated / painted . I don't use
non plated / painted in the 22's . Way too easy to go thru 300 rounds when more
than one shooter .

[ar15barrels]

I have been using RMR 124gr FMJ RN bullets along with Alliant Sport Pistol to reload 9mm rounds. I got the load data from Alliant's website and I'm basically using 4.3gr and an OAL of 1.125"(+/- 0.003").

I wanted to try RMR 124gr FMJ TC, but there is not load data for TC vs RN.
In theory, having the same bullet weight and same case capacity I should be able to use the same powder charge safely, right?

I measured the length of the bullets and the RN is 0.592" long while the TC is 0.557" long.
Is it correct (and safe) to shorten my OAL in 0.035" in order to have the same case capacity?

With the same bullet weight and construction, if you seat them to the same depth in the case (NOT overall length), you are leaving the same size case capacity and are probably OK.

[JagerDog]

All my semi autos have gas chambers . I only own 3 revolvers .
A shorter bullet does not matter as much in a revolver as in a semi .
plated and coated - more stuff in gas chamber if shorter .
My 22 lr semi's really build up stuff , but they are plated / painted . I don't use
non plated / painted in the 22's . Way too easy to go thru 300 rounds when more
than one shooter .

Funny. Most of my semi pistols are blowback.

How does higher pressure gunk anything up?

[tonyjr]

TAKE ONE OF YOUR CARTRIDGES AND RUB ON CEMENT . NOW ADD MORE
PRESSURE - THAT IS THE INSIDE OF BARREL . [ SORRY CAP LOCK IS STUCK ]

[JagerDog]

TAKE ONE OF YOUR CARTRIDGES AND RUB ON CEMENT . NOW ADD MORE
PRESSURE - THAT IS THE INSIDE OF BARREL . [ SORRY CAP LOCK IS STUCK ]

The casing is a gasket. See all that black schmutz down the side of the case in a low pressure load?

Higher pressure results in a good seal and more complete combustion.

22's are just dirty, period.

But chances are we're talking about a 9mm blowback pistol. There's no gas ports. There's a gas relief for blown primers or gas which makes it past the "gasket".

[NetworkSkullRipper]

With the same bullet weight and construction, if you seat them to the same depth in the case (NOT overall length), you are leaving the same size case capacity and are probably OK.

That is the case, both are FMJs from the same manufacturer with the only difference being the shape.

[NetworkSkullRipper]

The casing is a gasket. See all that black schmutz down the side of the case in a low pressure load?

Higher pressure results in a good seal and more complete combustion.

22's are just dirty, period.

But chances are we're talking about a 9mm blowback pistol. There's no gas ports. There's a gas relief for blown primers or gas which makes it past the "gasket".

Yes. These are all for 9mm blowback semis.

My intention is to keep the same amount of pressure.
But if I seat it for the same OAL, being the bullet shorter, the case capacity will go up making the pressure for the same powder charge drop.
Messing with the powder charge seems to be way more complicated than changing the seating depth to keep the case capacity constant.

[JagerDog]

Yes. These are all for 9mm blowback semis.

My intention is to keep the same amount of pressure.
But if I seat it for the same OAL, being the bullet shorter, the case capacity will go up making the pressure for the same powder charge drop.
Messing with the powder charge seems to be way more complicated than changing the seating depth to keep the case capacity constant.

"Messing" with powder charge isn't complicated.

That said, I think you're on a suitable path. See how they function in your firearm before making a large batch. Include a "plunk test".

Caveat: This assumes you aren't at "max" with the current bullet/powder combo. In that case, I'd reduce and work back up providing function was met with reduced.

[fguffey]

It is all doable, the part that is impossible for a reloader to understand is the contact point between the bullet and the bullet. The diameter at the lands does not change. If the reloader was able to determine the distance from the bolt face to the beginning of the rifling he can (should be able) to measure the distance for any bullet.

I use transfers, I transfer the dimension from the chamber to the seating die.

F. Guffey

[JagerDog]

How many tensions though?