I apologize if this has been addressed previously. I would like to load the 140 grain xtreme penetrators and get as close to underwood velocity as possible. It's too expensive to buy from them. Now I clearly want to do it safely and within pressure limits. I would like to know if anyone knows how they do it or has a load that gets there.
Thank you

Check the factory pull downs section.  I think there is a dissection in there.

Found it:

http://10mm-firearms.com/factory-10mm-ammo-pull-downs/underwood-140gr-lehigh-defense-xtreme-penetrator-pull-down/

11.8 grains of Longshot at a coal of 1.252"

This is not load data.  Work up very slowly.

Not even a clue about what the ballistics are on Underwood but here's Lehighs load data for it.
https://www.lehighdefense.com/pages/lehigh-defense-load-data-library

FWIW, they're showing a max load of 9.0 gr Longshot at 1315'ps out of a 4.6" barrel. COL at 1.250. If you do not know the difference between canister grade powder and non-canister powder, I'd suggest staying with the Lehigh data. Canister grade is trimmed for the burn rate to be more exact with regards to published loading data. Non canister assumes the loader, who will buy the powder in 5 ton lots, has a clue about developing loads via a piezo pressure instrument. What may appear to be Longshot in a round of Underwood might be non-canister Longshot with the burn rate completely different than the canister Longshot powder YOU can buy. You'll find that to be especially true of military ammo which is loaded by velocity, not by any specific weight of powder and will vary from lot to lot by weight.

That is a very good point and I do understand the difference. My goal was to safely approach their velocities. I didn't have specific powder in mind so I wasn't worried about duplicating their load. I spoke to Lehigh defenses rep about a week ago and we talked about their data. Falling that far short in velocity drops your energy by about 150ft/lbs +/-  At that point it almost isn't worth the effort. He also said they have no idea how Underwood gets there. I don't believe that's true   But from a legal standpoint I understand his statement and I would do the same in his position

Well after documenting all of these loads shown, it is clear that UW and others are exceeding the regular published loading data.  Not saying that the loads that are being sold are not still inside the SAAMI pressure MAP for 10mm.  But it clearly shows what was being used to get there.  Kevin has made the statement to me and others that his ammo is inside the SAAMI Pressure MAP.

These Pull-Down Documentations have been a labor of love for me to do.  As you look over the documentations you'll see that I follow a step by step plan to maintain accurate information.  People have been generous to share their ammo for these documentations.  Most being provide by Intercooler,  so I put his it back together and send it back for his further testing.  Some have provided rare ammo and I also at their request put theirs back as found and send it back due to them wishing to keep theirs but still share to preserve the history of these 10mm ammos.