Reloading 9mm FMJ versus TMJ


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RTMiller
April 7, 2011, 07:48 AM
If you had a recipe for a 124gr RN with a full metal jacket but you were using a 124gr RN with a total metal jacket, what adjustments (if any) would you make to the load?

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Siggie
April 7, 2011, 08:31 AM
I would start at approx 10% lower than the FMJ recipe and work up as needed.
But I think they are virtually the same if diameter and weight is equal.
FMJ only will have the lead core exposed at the bottom.

loadedround
April 7, 2011, 09:25 AM
Plus watch your OAL. Use your pistol barrel as a cartridge gauge.

bds
April 7, 2011, 09:48 AM
Depending on the diameter of the TMJ (if it's plated), you may need to use lower powder charge than FMJ load data. Generally, using start to mid range jacketed load data for plated bullets is a good idea.



FMJ is jacketed bullet (usually copper/brass disc punched to form the jacket) over soft lead core with exposed lead base (see bullet second from the left).

TMJ/CMJ bullets may be FMJ with round disk to cover the exposed lead base or copper plated lead bullets with plating covering the entire bullet with no exposed lead (see plated bullets below). If you see a round disk with at the base of the bullet, it's FMJ with a disk to enclose the exposed lead base (see bullet on the right below).

FMJ bullets with exposed lead base provide better obturation (bumping of the base to seal against the rifling/bore) for tighter bullet-to-barrel seal and more consistent shot groups. Some FMJ bullets even have concave lead base to improve obturation.

Most of your plated bullet diameters are sized between jacketed and lead bullets. Example is .355" for jacketed 9mm, .356" for lead and .3555"/.356" for plated (PowerBond and Berry's plated bullets - Rainier plated is sized at .355").


As to loading and performance, I find the FMJ with exposed lead base like Montana Gold provide the best performance (due to better obturation) and can be pushed to max jacketed load data. I typically load plated bullets using lead load data or low-mid range jacketed load data when lead load data are not available.

With JHP or plated bullets sized to jacketed diameter, bullet base may not obturate as much and leak more gas around the bullet base/rifling (this will cause chamber pressure drop and may affect accuracy). Some bullet manufacturers address this issue by concaving the bullet base to help obturate the base (see Montana Gold JHP and Rainier plated bullets below). I normally push these bullets to high-near max load data for more consistent obturation and shot groups.

With larger diameter plated bullets like Berry's and PowerBond, there's tighter bullet to barrel fit and less gas leak around the bullet base/rifling. This is the reason why Berry's recommend to use start-mid range jacketed load data. The tighter fit and less gas leakage translates to higher chamber pressure and more consistent shot groups.

Finally, Berry's offers Hollow Base plated bullets that's designed to obturate even better than regular plated bullets (see bottom picture). The hollow base and larger bullet diameter provides even tighter bullet base/rifling fit, even at lower powder charges. Here's a recent comparison hollow base bullet thread - http://www.thehighroad.org/showthread.php?t=581070

http://www.thehighroad.org/attachment.php?attachmentid=138364&stc=1&d=1300198575

http://www.thehighroad.org/attachment.php?attachmentid=139654&stc=1&d=1301693766

Siggie
April 7, 2011, 09:54 AM
Plus watch your OAL. Use your pistol barrel as a cartridge gauge.
Thanks for the addition Loadedround.. so easy and obvious that it is always forgotten to be mentioned (by me at least). :)

bds
April 7, 2011, 10:02 AM
Here's the loading specs I used for 124 gr Berry's bullets comparison - http://www.thehighroad.org/showthread.php?p=7217193#post7217193

RTMiller
April 7, 2011, 10:14 AM
Thanks for the replies.

FMJ bullets with exposed lead base provide better obturation
This was the issue I was thinking most about. I was guessing that a bullet without jacketing on the bottom would expand into the rifling better. But I didn't have a guess for how the pressures would differ.

Perhaps I should have posed the opposite to my question. If I have a recipe for a TMJ, would pressures rises significantly if I used a FMJ?

bds
April 7, 2011, 11:31 AM
I think that would depend on whether the TMJ recipe was for larger .356" plated bullet and how high the powder charge was.

If it was for tighter fitting Berry's bullet at start-mid range of jacketed load data, using the FMJ bullet that may not obturate may end up decreasing the chamber pressure.

For this reason, I usually push my FMJ/JHP bullets to high-near max load data.


Ultimately, you would need to range test your loads in your pistol/barrel to see which bullet/powder charge produces the most consistent chamber pressure/shot groups.

Holes on target speaks volumes and we could contemplate variables of reloading for days without reaching a definitive conclusion.

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