RepackboxTitan ReloadingADvertise hereRotoMetals2
Inline FabricationLee PrecisionMidSouth Shooters SupplyWideners

Results 1 to 15 of 15

Thread: Reloading M84 Trapdoor. Relationship between OAL / PSI/CUP / FPS

  1. #1
    Boolit Mold
    Join Date
    Nov 2019
    Location
    Colorado
    Posts
    10

    Reloading M84 Trapdoor. Relationship between OAL / PSI/CUP / FPS

    Hey all - after researching for an answer to my question, or a definitive direction, I decided to register and see what you all think. I'm sorry in advance for the wall of text. I also posted this over on the high road and got some much appreciated good info, but I'm casting a wide net where I can.

    I purchased a M84 SRC Trapdoor from TrapdoorCollector and I'm interested in loading smokeless for it. I bought some HSM "cowboy" loads for it, but they leaded the barrel something terrible. I purchased a box of HSM hard cast lead bullets from SportsmansWarehouse (.459, 405gr, single lube groove and loaded them over 24.3g & 25g of A5744) and they, too leaded the barrel. I've decided that perhaps the bullets @ .459 were too small, but they were absolutely too hard. The leading seems to start at the breech and walks forward towards the muzzle. It's leaded up and the groups walk out after less than 5 shots. By 10 shots the group is LARGE.

    I slugged the bore and got .460+/-.

    The factory HSM cowboy loads measured at OAL 2.535.
    The max OAL to the lands with the HSM bullets for my reloads measured at 2.585.
    I loaded the HSM bullets to a 2.580 OAL. Not a problem, that's more than the min OAL listed for the 5477 powder I was using, so I knew I was well under (ideally) pressure limits at the min load.

    I've since bought a couple more types of bullets to try and sort the leading out. The two that arrived today have me wondering about OAL, pressure levels, and what's safe...

    I bought two models from GTBullets (nice people - great communication and friendly) - a 390gr FlatPoint HollowBase and a 425gr FlatPoint flat/solid base, both cast @ .460. They use a 96-2-2 alloy, which seems plenty soft (can dent with a HB pencil - I couldn't even scratch the HSM bullets with same pencil), and each have two lube grooves.

    My issue comes with the OAL when I seat the bullet(s) just touching the rifling (using cleaning rod method).

    For the 425gr, I get a OAL @ impact of rifling of 2.4575.
    For the 390gr, I get a OAL @ impact of rifling of 2.5225.

    For the 390gr the closest load for A5744 in my Lee manual is the 405gr - which shows a min OAL of 2.550 at the "never exceed" load (@16.1kcup). I want to load .020 off the lands, so that puts my OAL of a finished round at 2.5025, which is .05 shorter than the min OAL listed (2.550). To compound my concern, the 390gr is 1.0295 long, whereas the 405gr HSM was 1.009. The longer bullet will be seated further in the case AND is .02 longer, meaning even MORE lead in the case and less open case volume (HOWEVER - there is a 385gr bullet listed with a 2.505 min oal @ max charge for a LOT of other powders, so I'm not as concerned with this bullet as the 425gr I list next).

    For the 425gr there is a 420gr A5744 load in the Lee manual, but the min OAL is 2.600 (@16.1cup). Additionally, the 425gr measures 1.0345 long vs the HSM @ 1.009. (Again, the longer length (to me) means not only is the OAL shorter putting the bullet further back in the case, the bullet is deeper in the case than the HSM would've been at the same OAL because the bullet is longer.) If I go .020 off the lands, it puts the OAL for the 425gr @ 2.4375 - a LOT less than the "min oal" for the never exceed load.

    I want to start on the LOW end of the scale, and try and get a consistent, accurate load between 1150-1250fps with each of these bullets. I'll pick the most accurate load within that velocity range, if more experienced folks think that's a safe approach. To be clear, I'm using fps waypoints because I don't have any other way to gauge pressure, and the concern I have is that the shorter OAL is skewing the pressure curve in a meaningful way. I need a safe start - and stopping - point.

    Because my OALs are SHORTER than published for max loads (most notably the 425gr @ .1625 shorter), I plan on starting the 390gr projectile @ 22gr (405gr min is 24.3) and the 425gr projectile @ 19gr (420gr published min is 25.6). I came up with those starting points by reviewing what AA does to the 5744 powder charge at differing OALs in the 45-70. It's rough, but it's the best correlation I could find.

    Last note - if I set either bullet crimped at the groove, I end up in the rifling pretty good, although then the COL issues I have may be a non-issue, I'd then be trading the COL issues for the bullet set into the lands pressure spike issue. As I related on another forum, I don't have experience with that other than what I've read.

    Here's the questions...

    1) Using A5744 (or H4198 which I also have on hand), is the shorter OAL a point of concern for an original TrapDoor at minimum load levels as long as I a) stay well away from "never exceed" loads and b) monitor velocity during testing and keep a reasonable goal of 1150-1250fps?
    2) Can I assume, for my practical purposes, that given the "known" weight of the projectiles and moderate FPS goal, that I can develop safe loads even with the shorter OAL using FPS as my waypoints?
    3) If FPS can work, should I adjust my "target", or is 1250 reasonable for a high end? (… understanding I may settle on a load slower than that depending on which load tests out most accurate.) Could 1150 for 425gr and 1250 for 390gr be those reasonable high end target velocities?

    Who knows - after all this contemplating, they may not shoot well at all, and that I can handle. I just want to be safe while I work through this.

    Thanks all.

    Edit: I also sent a check off for 150 bullshop 405gr hb lee cast to my barrel dimensions per his recommendation. These loads are only for the GT bullets - as the bullshop bullets should be fine with the factory lee & accurate loading data.

  2. #2
    Boolit Master
    Chill Wills's Avatar
    Join Date
    Oct 2011
    Location
    Colorado
    Posts
    2,263
    I di not read every word because I have to run. But did scan it. I am starting again with my interest in the Springfield 1884 again to. I only shoot black powder so my input will be from a different angle. I have shot a half dozen NRA 500meter Silhouette matches with one more then 10 years ago and have some good experience.

    Just the one thing that caught my eye is OAL. You can seat longer than you are and the round will push into the rifling. Play with it.
    Lighter than 500gr bullets will print higher than the sight markings as will loading with smokeless powder. That is not a bad thing if you sight in and record your settings. About 200yds should be your starting range to work up loads with that sight.

    I will post more after reading (each) word of your post

    It is good to see your level of interest!
    Chill Wills

  3. #3
    Boolit Mold
    Join Date
    Nov 2019
    Location
    Colorado
    Posts
    10
    Thanks Chill -

    I'm good with pushing into the rifling so long as I am sure I am at a "safe" starting point. They don't make these rifles anymore IIRC, I crimped one at the groove ( but didn't measure OAL at that time ) and it took a little bit of leverage for the last 1/4-1/8 or so of closing the breech block. When I opened the block, the case sprang free just fine, and the lands had imprinted/scraped into the bullet cone.

    Click image for larger version. 

Name:	20191117_152819.jpg 
Views:	19 
Size:	26.6 KB 
ID:	251406
    Click image for larger version. 

Name:	20191117_152747.jpg 
Views:	18 
Size:	23.4 KB 
ID:	251407

    I took care of the front sight with a copper penny and some finesse - I know it's "simple" but I was proud when I finished. Once I get a reliable/repeatable load, I'll file it to zero @ 100. The original front sight is safe and sound, and easily pinned back in position.

    Click image for larger version. 

Name:	Tall Sight.jpg 
Views:	30 
Size:	46.4 KB 
ID:	251382
    Last edited by CoRifleman; 11-17-2019 at 08:24 PM.

  4. #4
    Boolit Master


    Kraschenbirn's Avatar
    Join Date
    Dec 2006
    Location
    East Central IL
    Posts
    2,804
    If you can find a copy, Spencer Wolf's 'Loading Cartridges for the Original 45-70 Springfield Rifle and Carbine' is the best source I've found for the Trapdoor. Loading info is, primarily, for BP but book covers history, cartridge development, ballistics, sights, etc. Btw, that Lee boolit you're getting from Bullshop was developed by Spence Wolf as a copy of the original 1873 Trapdoor round.

    I'm currently feeding three .45-70 single-shots, including a Trapdoor carbine, and have found the current Lyman 'Cast Bullet Handbook' to be an excellent source for smokeless data. My 5744 loads chrono within 50 fps of their numbers when fired from my 1885 Highwall and print under 2 MOA at 200 yds...off cross-sticks using iron sights.

    Bill
    "I'm not often right but I've never been wrong."

    Jimmy Buffett
    "Scarlet Begonias"

  5. #5
    Boolit Mold
    Join Date
    Nov 2019
    Location
    Colorado
    Posts
    10
    Quote Originally Posted by Kraschenbirn View Post
    If you can find a copy, Spencer Wolf's 'Loading Cartridges for the Original 45-70 Springfield Rifle and Carbine' is the best source I've found for the Trapdoor. Loading info is, primarily, for BP but book covers history, cartridge development, ballistics, sights, etc. Btw, that Lee boolit you're getting from Bullshop was developed by Spence Wolf as a copy of the original 1873 Trapdoor round.

    I'm currently feeding three .45-70 single-shots, including a Trapdoor carbine, and have found the current Lyman 'Cast Bullet Handbook' to be an excellent source for smokeless data. My 5744 loads chrono within 50 fps of their numbers when fired from my 1885 Highwall and print under 2 MOA at 200 yds...off cross-sticks using iron sights.

    Bill
    I appreciate it, Bill, thank you.

  6. #6
    Boolit Grand Master


    Larry Gibson's Avatar
    Join Date
    Jul 2005
    Location
    Lake Havasu City, Arizona
    Posts
    17,538
    CoRifleman has been PMing me with the questions also. Here are 3 of my responses which may prove helpful to others reading his ponderings:

    I’ve been mulling over your post a bit because you threw a lot of info in there. That's ok, just needed to sort it out so I understood correctly.

    First of all, don't over think the OAL. I seat all my TD loads so the driving band is engraved by the leade when the breach block is closed. Any rise in psi doing so is negligible and actually not measureable. Also the volume of the case is such that slight variations in seating depth have little effect.

    The HSM bullets are too hard, too small in diameter and the lube sucks. I would not use tem and just chalk it up to experience.

    Quite frankly A5744 is a poor powder to use for the low end loads you seek. The H4198 would be a better choice with the solid base 425 gr bullets. I suggest you start at 21.5 gr and use a 1 1/2 gr Dacron filler. Work up to 27 gr in 1 gr increments. That should work you up to 1200+ fps. Your rifle will tell you what load is giveing the best accuracy. If you really want to try the A5744 then start at 22 gr and work up to 26 also using the Dacron filler.

    If the 390 gr HB has thin skirts to expand into the rifling then I do not suggest either powder (A5744 or H4198) as they will have too much exit pressure and will "blow the skirt out". I've use Montana soft cast (20-1) HB bullets and found Unique to be the best powder. No filler is used as that will plug up the HB or cause uneven distortion on muzzle exit destroying accuracy. Besides, with the faster burning Unique and that weight of bullet a filler is not needed. Start at 10 gr and work up to 13 gr in 1 gr increments. Accuracy will go south all of a sudden which tells you the skirt is getting blown. Back off 1 gr and there's the load for that bullet.

    The skirt on the HB bullet your using is very similar to the M1873 bullet and the Lee copy of it. Contrary to the ad hype you will not get any "bump up", even with that softer alloy at TD pressures.

    Also regardless of the ad hype A5744 is very position sensitive at the lower psi and velocity you want to use n the TD with 400 +/- gr cast bullets. I suggest with that HB bullet seated to the crimp groove with a roll or very mild crimp working up to 25 gr A5744 or 25 gr H4198 with a 1 gr Dacron filler. Velocity should be at or under 1300 fps with a psi of 21 - 22,000 psi. Using the Lee 405HB bullet (cast of 16-1 alloy, 207 gr) that load runs 1790 fps at 22,400 psi as actually measured. That should also be a good load with the 425 gr FB bullet as I use it with 420 gr cast commercial bullets in my H&R OM. Not my preferred load but when I acquire some 5744 that's where I use it. The SAAMI MAP for TD level loads is 28,000 psi so you've a good safety margin.

    The previously suggest loads for H4198 is still good also, just use the Dacron filler.

    As to an increase in psi seating against the leade, that applies to jacketed bullets. Does QL say that for cast also? Pictures look fine, especially if you just work up the load.

    Many of the pressures you'll find listed for the 45-70 and other older cartridges are simply figures based on either CUP or LUP pressure measuring systems. Those figures were referred to as "psi" back when they were the only systems used. "Psi" used today is based on transducer/strain gauge measuring and is different than CUP or LUP. Also there is no reliable method of converting CUP into psi as used today other than a direct comparison based on measurements of a single load and seldom do we see that listed anywhere. The MAP pressure I quote from SAAMI (28,000 psi) is based on transducer/strain gauge measurements. The measured psi I gave you are those taken myself with an Oehler m43 PBL using my own test rifles. The loads I gave you are perfectly fine for a TD.
    Larry Gibson

    “Deficient observation is merely a form of ignorance and responsible for the many morbid notions and foolish ideas prevailing.”
    ― Nikola Tesla

  7. #7
    Boolit Mold
    Join Date
    Nov 2019
    Location
    Colorado
    Posts
    10
    Yes, Larry has been a huge help here.

    I'm moving away from being overly concerned with col or oal in this scenario (large case, starting slightly below published loads), and he's provided valuable start load info for me.

    I've loaded the 390gr hollow base to a slight taper crimp at the crimp groove (which puts me above min oal, anecdotaly), which in turn sets the nose into the rifling slightly. I loaded them with 5744 at 21-25gr in 1gr increments. I don't have any dacron, so that will come next time.

    I was able to look, off-angle, at the rifling near the chamber and it is MUCH more pronounced than near the muzzle. Such is the life of a 130yr old rifle, I suppose.

    Excited to see how these do.

  8. #8
    Boolit Grand Master

    Wayne Smith's Avatar
    Join Date
    Mar 2005
    Location
    Hampton Roads, Virginia
    Posts
    11,194
    Have you measured the barrel at the muzzle vs at the chamber? I'm curious if the barrel is worn enough to effect either accuracy or leading. But I believe you stated that the leading started just above the chamber?
    Wayne the Shrink

    There is no 'right' that requires me to work for you or you to work for me!

  9. #9
    Boolit Master Drm50's Avatar
    Join Date
    Jun 2016
    Location
    SE Ohio
    Posts
    1,477
    I've had several original trap doors and shot BP & Unique both. I got old & lazy I went to 2400/ Dacron filler with 322gr cast at 1100fps.

  10. #10
    Boolit Master
    Join Date
    Aug 2005
    Location
    Near Mazomanie, Wi.
    Posts
    1,080
    In my opinion, your boolits are too small in diameter and too hard as well. The boolits should be .001-002" over bore diameter. 5744 was developed as a substitute for black powder in the old timers. Do a search for Paul Matthews, he has done extensive testing and is a BPCR shooter. He also does not recommend fillers in the TD.
    NRA Life
    NMLRA Life
    F&AM

  11. #11
    Boolit Mold
    Join Date
    Nov 2019
    Location
    Colorado
    Posts
    10
    Quote Originally Posted by Wayne Smith View Post
    Have you measured the barrel at the muzzle vs at the chamber? I'm curious if the barrel is worn enough to effect either accuracy or leading. But I believe you stated that the leading started just above the chamber?
    At the LEAST it starts in the middle, but definitely not at the muzzle.

    I suppose I could drive a slug in the muzzle to halfway and then back out using short dowel sections inserted at the receiver. Can that work? If the dimension is appreciably different, then the throat is the tightest part of the bore I guess.

  12. #12
    Boolit Master
    Join Date
    Sep 2016
    Posts
    1,073
    Slug your barrel to find out what your dealing with. Trapdoors cab have up to .463 bore dia. Anything sized less than that will give poor accuracy What ever number you get from slugging go .002" over that number. The better the bullet fits the bore the better the accuracy. Frank

  13. #13
    Boolit Mold
    Join Date
    Nov 2019
    Location
    Colorado
    Posts
    10
    Quote Originally Posted by samari46 View Post
    Slug your barrel to find out what your dealing with. Trapdoors cab have up to .463 bore dia. Anything sized less than that will give poor accuracy What ever number you get from slugging go .002" over that number. The better the bullet fits the bore the better the accuracy. Frank
    Frank, I did slug it, and got low .460X's (.4600-.4604... Closer to .460 than .461). I think the question was if possibly the throat is much tighter than the muzzle end. I do appreciate the input on bullet dia, though! I'll take these soft .460s I've loaded to the range and try some larger as I get them. Thank you.

  14. #14
    Boolit Master



    skeettx's Avatar
    Join Date
    Jul 2009
    Location
    Amarillo, Texas
    Posts
    3,197
    At that bore diameter, load the largest diameter bullet that will allow chambering.

    Mike
    NRA Benefactor 2004 USAF RET 1971-95

  15. #15
    Boolit Master
    bigted's Avatar
    Join Date
    Mar 2009
    Location
    Sweet Home Oregon
    Posts
    4,077
    My old military's always get the same treatment.

    Shoot several cartridges ... take these fired cases and gently remove any of the remaining crimp ... now measure the mouth/neck of all the cases fired in the rifle ... take these measurements and get your average size. This is the size I shoot in that rifle. Next begin swapping primers, boolits, powder and grainage till you arrive at the most accurate load for your rifle.
    WHEN ALL IS SAID AND DONE...MORE WILL BE SAID THEN DONE

Posting Permissions

  • You may not post new threads
  • You may not post replies
  • You may not post attachments
  • You may not edit your posts
  •  
Abbreviations used in Reloading

BP Bronze Point IMR Improved Military Rifle PTD Pointed
BR Bench Rest M Magnum RN Round Nose
BT Boat Tail PL Power-Lokt SP Soft Point
C Compressed Charge PR Primer SPCL Soft Point "Core-Lokt"
HP Hollow Point PSPCL Pointed Soft Point "Core Lokt" C.O.L. Cartridge Overall Length
PSP Pointed Soft Point Spz Spitzer Point SBT Spitzer Boat Tail
LRN Lead Round Nose LWC Lead Wad Cutter LSWC Lead Semi Wad Cutter
GC Gas Check