9mm case capacity w/ JHP in CZ

Relatively new to reloading, especially 9mm: I recently received my 9mm RMR 124 gn TCFP match winners and 124 gn JHP wanted to run my thought process by those more experienced:

I’m using Win231, have had great success with 4.3 gn behind Berry’s RNFB at 1.14” in CZ P01. Got the RMR’s behind good reviews on here and elsewhere and trying to create a similar nice target load.

My manuals (Lee 2nd, Lyman 50th) don’t show much info on TCFP, so loaded ladder based on plated 124 gn at 3.9, 4.1, 4.3 gn . I go in 0.2 gn increments as my Lee powder scale throws 231 in 0.1 +/- 0.04 gn, so I don’t see the benefit in comparing e.g. 4.14 vs 4.16 and I’ll stop my ladder at 0.1-0.2 below max.

the Lyman 50th shows 124 gn JHP of 3.8 -4.5 of 231 with COAL of 1.06”. Reading on forums, RMR JHP are needed to loaded pretty short in CZ’s shorty throats, but also have a pretty long ogive. Plunk test has TCFP at 1.09 COAL and JHP at 1.05” (which even then is just at the go range based on a few dummies).

What concerns me is I started looking at my brass length 0.748” (untrimmed now once fired Win) and bullet lengths. Even though the TCFP is 0.05” less than the berrys, actual case capacity based on seating depth (as length) is only reduced by 0.012” compared to Berrys and a similar bearing surface so I feel pretty comfortable expecting similar results to the Berrys RN. The JHP however reduces the case capacity by 0.056” and has a longer bearing surface, both which lead me to think of higher pressure. On top of that, my brass seems to be 0.06” below the reloading books.

My question is if I load my ladder at 3.9, 4.1, 4.3 for the JHP, could I potentially have a dangerous pressure spike at the 4.3 or even 4.1 given the RMR may have different dimensions than the test projectile in Lyman? I’ve read a 0.03” seating difference can greatly spike pressures and it seems that different manufactures could produce JHP’s at different lengths. I am using federal SP match primers and in theory know what to look for but I my knowledge have not experienced an over pressure load to date. I am ok with learning what overpressure looks like, I just don’t want to lose appendages or a gun while doing so.
Thankyou.

Thank you both. Maybe I’ll add 3.7 gn as the first load. Peels, what is the case length of the brass you are loading the 4.3 in. My true target COL is 1.052-1.054”.

LordpaxMan: unfortunately, I do not have access to a chrono. The HDY HAP sure have low loads and low velocity. Do you think it is strictly the shorter COL or some other feature of the projectile?

edited to correct COL typo

This is all great stuff. I’ll check feeding of the dummies and load some tonight and tomorrow with five of each at 3.7, 3.9 then step up to 10 each at 4.1 and 4.3 gn. I’ll be headed to the range Sunday and let you know how it goes (first outdoor trip of the season in the tinder box that is CA).

Unfortunately with no chrono data, but if there are an abnormal amount of typos, I may not have the full set of appendages

231 meters relatively well in the Lee Deluxe perfect powder measure, but if I throw 15-20 at 4.3, I typically will get 1 or 2 in the +/- 0.04 range and bell curve to an average of 4.3 gn. My other 9mm/357 powder is unique but that is all over the place. If I’m going to improve one thing, I’d think it safest to get a better powder thrower. What would you all suggest?

Went out last Sunday, didn’t get to do much more than a quick 5 shot ladder on each since my friends children joined and it doesn’t make it enjoyable for new shooters watching someone slowly fire shots at paper, mark the targets, collect brass inspect, etc. they had a blast shooting some old soda cans with their air rifle though and “helping” me find my brass.

What I did find was 4.2 gn W231 with 1.09” RMR TCFP MW and with1.05” RMR JHP MPR seemed to give the best groups at 25 ft offhand, but as it was the first range session of the “don’t start a fire” season, I may have been improving as I put more rounds down range (and I’d never call myself a marksman anyways).

Although the 4.2 gn with JHP grouped best, the 4.3 gn had an interesting pattern, all about 3/4” left of POA, but they were in a vertical line you could intersect all with a pencil. I plan to load more of 4.2 vs 4.3 to expand the test.

I did mark ejection distance for each group of brass and the 3.7 gn landed about 5 ft away, with each tenth adding about a foot until 4.0 gn/8 ft and that seemed to be the limit for both TCFP & JHP (installed CGW pro kit with factory recoil spring). All loads functioned perfectly with the TCFP being overall more consistent in grouping, but was my second ladder.

pic attached is the JHP fired brass starting at 3.7 on the left, bump to 3.9, then 4.0, 4.1, 4.2, and ending with 4.3 gn on the right with three unfired rounds at the bottom (5 rounds each, but only could confirm charge on the ones showed). No noticeable pressure signs, with some minor flattening of the depth of dimple starting at 4.0. All brass is 1-2x fire Winchester from factory win service grade 115 gn FMJ.

All loads felt lower recoil than factory with better groups even though factory was fired later. 4.3gn win231 w/ berrys RNFB (previously worked up load) grouped very nicely and may edge out both the TCFP and JHP, but I plan to pit them all against each other in a higher round count trial in the future.

thanks again for all the replies, been a shadow lurker reading posts in the last 1.5-2 years and along with the manuals; this and other forums have been extremely valuable. Hope I will be able to help someone in the future like you all have helped me.