Myth busting OAL/COL - Long vs Short

[LiveLife]

Does loading longer reduce gas leakage and produce greater accuracy?

I have seen both sides of this question where 40S&W 180 gr bullets loaded longer than SAAMI max of 1.135" to 1.142" with MBC lead/Berry's plated TCFP and 1.155" with RMR HCM RNFP produced greater accuracy while 9mm 115 gr FMJ/RN bullets produced greater accuracy at shorter 1.130" - https://www.thehighroad.org/index.p...ick-plated-bullets.761471/page-4#post-9645513

While continuing load development and range testing with IMR's new Red, Target and Green powders along with Vectan Ba 9.5 and Ba 9 powders, I wanted to start verifying accuracy difference between long vs short OAL/COL.

Since 4.2-4.3 gr of IMR Target produced smaller groups with RMR 115 gr FMJ/HCM RN and 124 gr HCM RN bullets, same powder charge was used to load following test rounds - https://www.thehighroad.org/index.p...-and-vectan-ba-9-5-ba-9.817796/#post-10508215

- RMR 115 gr FMJ at 1.130"
- RMR 115 gr FMJ at 1.155"

Sorry for the wet target pictures but it was a rainy windy range day and plastic sheet over the target paper did not do much to keep the paper dry. Comparison shot groups should give us a good starting point for this myth busting thread.

17" Just Right carbine was used with Bushnell 6-18x50mm scope and shot from inside the suburban (my rain shelter) using UTG bipod at 25 yards for 10 round shot groups.

RMR 115 gr FMJ with 4.2-4.3 gr IMR Target at 1.130" OAL (Left) and 1.155" OAL (Right)

Same 1.130" shot group with different 1.155" shot group

 

[Dudedog]

Interesting, so the shorter OAL appears to have shot better.
It makes me wonder was the shorter load better because it may have had more grip on the bullet allowing pressure to build more, because of the possible velocity difference (guessing maybe 25-35 fps or so for .025 (but I may be way off), bullet jump or some other reason.
Can't wait for more results, it should be interesting.
Thanks for testing and posting for us on this one and all the others.

 

[sbwaters]

I would have thought -- and who am I to think with such limited expertise -- that the issue is not OAL but rather what a particular bullet/powder/case/barrel happens to like. The harmonics of any combination are going to be different and not transfer to another gun, bullet, and/or powder.

 

[LiveLife]

There are many reloading variables that will affect how chamber pressures build/peak and as already discussed many times in various "reloading variables" threads, some reloading variables may overshadow other reloading variables.

Ultimately, more efficient powder burn will produce more consistent chamber pressures which in turn will produce more consistent muzzle velocities that will produce smaller shot groups on target.

With 40S&W 180 gr bullets with longer bullet bases, using longer than SAAMI max OALs has produced smaller groups in different barrels likely from less gas leakage overshadowing reduction in neck tension. But with 9mm 115 gr bullets with shorter bullet bases, using shorter OALs has produced smaller groups in different barrels likely from increased neck tension overshadowing less gas leakage. Perhaps that's why Atlanta Arms, who supply various national match teams including Army Marksmanship Unit/Marine Service Pistol teams, uses shorter 1.130" OAL for their 9mm 115 gr FMJ Match AMU ammunition which has to produce less than 1.5" 10 shot groups at 50 yards - https://atlantaarms.com/products/9mm-115gr-fmj-match-amu.html

But that's why we do these "Myth busting" threads - to test and verify our reloading theories and notions. I do believe holes on target speak volumes and repeated range tests should provide us with sufficient enough verification.

17" Just Right carbine uses 1:16" twist rate barrel. I plan to repeat the testing with 16" PSA carbines with 1:10 twist rate barrels.

BTW, 25 yard 10 shot groups of RMR 115 gr FMJ and 115 gr Hardcore Match RN bullets loaded to 1.130" OAL with 4.2-4.3 gr of IMR Target - https://www.thehighroad.org/index.p...-and-vectan-ba-9-5-ba-9.817796/#post-10508215

 

[Bart B.]

The harmonics of any combination are going to be different and not transfer to another gun, bullet, and/or powder

I disagree if you're referring to the firearm's harmonics.

The parts of the firearm has the same shape and weight regardless of the load shot. It's resonant frequency and harmonic multiples are the same for each shot because those two physical properties are the same for every shot. Those vibrating parts' amount will change; frequencies stay the same.

 

[LiveLife]

Interesting, so the shorter OAL appears to have shot better.

It makes me wonder was the shorter load better because it may have had more grip on the bullet allowing pressure to build more, because of the possible velocity difference (guessing maybe 25-35 fps or so for .025 (but I may be way off), bullet jump or some other reason.

As I mentioned in Post #4, at least for 9mm bullet with shorter base. 115 gr FMJ loaded shorter at 1.130" producing smaller shot group is likely from greater neck tension producing more consistent chamber pressures which overshadowed effects of less gas leakage.

Due to heavy rain, I couldn't set up the chrono but will capture chrono data on the next range trip.

Keep in mind how short you can go with the bullet will depend on the density/fluffiness of the powder you are using and you should always calculate the bullet seating depth and max case fill so as to not compress the powder charge.

 

[azrocks]

Keep in mind how short you can go with the bullet will depend on the density/fluffiness of the powder you are using and you should always calculate the bullet seating depth and max case fill so as to not compress the powder charge.

First off, thanks for the work and your findings!

Noob question - why is compressing the powder charge bad?

And other question - It would seem to me that in order to get the most consistent powder burn, the optimum load be one where the powder came right up to the base of the bullet (either compressing slightly or just before it begins to compress). Is my thinking in error, and if so why?

 

[Blue68f100]

I don't thing you have much harmonics in a handgun. With a 4" barrel I dought that it has much impact, rifle length yes. Then it all depends on the handgun being used. Some have lockup barrels so everything is tight, others do not. I've only had 1 hand gun where adj the OAL had a impact. I went shorter like you did and my groups reduced by 1/2. Now remember in the 9mm and 40 cal a OAL change shorter of 0.020" can greatly increase the pressure of these rounds, particularly with some powders. SO I would suspect a high pressure had more impact than bullet tension.

 

[rsrocket1]

bds,
When you do chrony the loads, I have a feeling that your shorter rounds will also demonstrate lower SD's and greater consistency in MV.
Higher neck tension may contribute, but you also have higher pressures. With most of my loads, higher pressures with more powder also produce better consistency in MV so higher pressures due to shorter lengths may also do the same.

 

[LiveLife]

There was a break in the rain so I took a quick range trip.

Promo load bullet seating depth was calculated to 1.135" instead of 1.130" to not compress powder charge. Glock 22 and KKM conversion barrel was used for chrono testing:

- IMR Target 4.2-4.3 gr RMR 115 gr FMJ 1.155" OAL: 1036-1068-974-1044-1015 fps - SD 35
- IMR Target 4.2-4.3 gr RMR 115 gr FMJ 1.130" OAL: 1072-1045-1078-1059-1077 fps - SD 14

- Alliant Promo 4.1-4.2 gr RMR 115 gr FMJ 1.155" OAL: 1093-1027-967-1095-1126 fps - SD 64
- Alliant Promo 4.1-4.2 gr RMR 115 gr FMJ 1.135" OAL: 1102-1092-1072-1142-1144 fps - SD 31


25 yard 10 shot groups from 17" Just Right carbine shot with UTG bipod and Bushnell 6-18x50mm scope:

IMR Target 4.2-4.3 gr RMR 115 gr FMJ 1.155" (Left) and 1.130" (Right) - Except for one flyer, 1.130" load on right formed a tighter shot group. This is third range test for IMR Target which supports tighter groups with shorter OAL using RMR 115 gr FMJ. (Splatters on targets from rain soaked rocky backstop and elongated holes from target cardboard leaning back)

Alliant Promo 4.1-4.2 gr RMR 115 gr FMJ 1.155" (Left) and 1.135" (Right) - Promo loads were less conclusive and will conduct further testing.

 

[jell-dog]

bds,
Very interesting thread!
One question tho - as an example, if my reload manual says 115gr Jacketed RN as 4.2gr to 5.2gr of X-powder at 1.160" COL, how do I determine a safe range of X-powder when I reduce the COL to 1.130"COL?

Keeping in mine that I would check to see if the charge I would use would result in a compressed load.

Thank you,
JD

 

[lauderdale]

Measure case fill/load work up?

 

[rsrocket1]

bds,
Your low SD's on the shorter bullets make sense because you are shooting at about 2,700 psi higher peak pressure (QuickLoad estimation). Now, the difference between 34k and 37k shouldn't mean a more "complete burn" of Promo because the powder is 100% consumed in less than an inch of bullet travel from the case. Maybe it does. The peak pressure is reached at 0.21" of bullet movement with the 1.155" COL load and 0.20" with the 1.135" but 100% propellant consumption is 0.929" vs 0.834". Of course these are not "gospel" numbers, but the relative differences could hold a clue as to why the higher pressures lead to a more consistent MV even though you would think you'd reach a limit and then get no further improvement. You'd think a fast powder like Promo would have reached that limit above 20kpsi. Maybe not.

In general, I like to load my mid range bullets shorter than the standard 1.135" in 9 (1.05") and 40 (1.10") simply because that's where the conical section of the TC bullets turn "vertical" and I seem to get more reliable feeding that way. I do have to back off from "max published loads" simply because I know the pressures will be greater when the case volume is less. If I want more velocity than a fast powder can give me with my cartridge dimensions, I go to a slower powder and use more.

 

[LiveLife]

One question ... if my reload manual says 115 gr Jacketed RN as 4.2 gr to 5.2 gr of X-powder at 1.160" COL, how do I determine a safe range of X-powder when I reduce the COL to 1.130"COL?

Keeping in mine that I would check to see if the charge I would use would result in a compressed load

For me, it's always determine the Working OAL/COL first then conduct powder work up (and then reduce OAL further for even more accurate load). If published OAL is really long like 1.160" and my working OAL is short like 1.125"-1.130", I may reduce start/max charges by .2-.3 gr depending on the powder.

Regardless what published OAL is listed, I always do the following for load development:

- First determine the Max OAL (Say 1.175") then Working OAL (Say 1.160") that will reliably feed from magazine
- Using 1.160", conduct full powder work up to identify charge with smallest shot groups
- Then incrementally decrease the OAL by .005" and look for accuracy trends after checking for powder compression
- If accuracy doesn't improve, I will use 1.160"
- If accuracy improves with shorter OAL like 1.130", I will use 1.130"

 

[rfwobbly]

Interesting, so the shorter OAL appears to have shot better.
It makes me wonder was the shorter load better because it may have had more grip on the bullet allowing pressure to build more....

Ala NASA allowing rockets to leave the launch pad only after the engines reach a certain energy via explosive bolts. Or, it could be as simple as deeper seating gives the bullet more structural support so that it's not knocked off-axis when the cartridge slams into the feed ramp.

I have replicated these same results in 9x19. No definitive answers, other than shorter OALs do perform better. However, in stead of phrasing this research in terms of OAL, I think it better to talk about this in terms of "seating depth". That is, how much of the bullet engages the cartridge case.

 

[rsrocket1]

Agree 100% rfwobbly. Seating depth (or case volume) is the critical factor here although when using the same bullet, shortening the COL is the same as increasing seating depth.
A while back, I made an excel graph showing the relationship between seating depth, peak pressure and muzzle velocity with a hypothetical 9mm bullet and constant powder charge. You can see that the MV is somewhat linearly, but the pressure goes up exponentially as you increase the seating depth. That's why a chronograph reading is not a good indication of whether a shorter COL is safe. With pistol cartridges, neither is reading primer flatness as some primers won't flatten until you are in dangerous rifle pressure territory (>55,000 psi)

 

[GooseGestapo]

Something over looked is the effect of seating deeper, improving axial alignment of the bullet, or in other words, the "runout" of the bullet. Bullets that engage the rifling "crooked" or out of alignment, will "wobble" in flight, and diverge from the bore axis like a baseball pitchers curve ball.
However, a long shank bullet like a .40 180gr, will have sufficient seating depth to ensure alignment and seating to near the grooves will further reduce misalignment to improve precision. Most 115 or 124gr 9mm RN have very short full diameter bearing surfaces. The 147 being better in this regard.
However, these make more difference in a sloppy service grade chamber, vs. a tight match grade chamber/barrel.