What actually determines OAL?  As long as a complete round passes the "plunk test" in your specific pistol, will it be safe?

In a pistol?  Mags and chamber.  I load my .40 and 9mm long, and they are both fine.

Plunk test is a poor test of cartridge reliability, very poor. Plunk test should never be a stand alone performance test.

That said, I'm gonna step back and watch your thread run before painting the big picture.

Later,

dc.

The bullet seating operation determines the OAL of the cartridge.  In this operation, it is the shape of the bullet (the nose) and the shape and setting of the seater stem (depth of the seater stem within the seating die) that determines what the final length of the cartridge will be.

Do you intend to ask what determines the maximum allowable OAL for a given firearm?  If so, it's either the magazine's dimensions or the barrel's throat and lead dimensions.  The shape of the specific bullet type is also a factor.  The bullet's shape determines the OAL the bullet protrudes far enough beyond the throat and lead to engage the rifling.

Is your "plunk test" a quick check to determine whether the cartridge is headspacing off the case rim (as it should) or whether  the bullet is engaging the rifling?

Max OAL for a cartridge is set by SAAMI based on the magazine or cylinder dimensions.   In certain situations you can ignore this and load longer, (e.g. loading for a single shot .44 magnum you would not need to stay below the SAAMI max length which is intended to fit in a S&W revolver.

Max OAL for a given bullet/cartridge may also be constrained by the bullet ogive.  A flat nose projectile is necessarily going to have a shorter OAL than a round nose if they are both going to feed from the same magazine.

Minimum OAL is usually only relevant in straight walled pistol cartridges, and is based on the test and load parameters of the bullet powder combination tested.  It's important not to go below minimum OAL (especially on near max loads) since very small changes in seating depth can cause a drastic and dramatic affect on cartridge pressure.  Hornady had a chart a while back where they showed the pressure increase from seating a 180 grain .40 S&W too deep.  They achieved 30-06 pressure levels (in a test barrel obviously) by seating the bullet too deep by about .100"

Ok, here comes a bomb. Chew it anyway you'd like and PLEASE CHALLENGE this statement. It's somewhat off topic but trust me to beeline to original post for detailed, informative response.


No amount of setback will cause a handgun to blow apart.


Now, let'em rip.

Yes, I'm for real,


dc.

Both Sig Sauer and Glock released 'bulletins' to LE agencies a couple of years back due to bullet set back problems.

It seems that some officers were prone to coming home after their shitfs, removing their magazines, ejecting the chambered round and reinserting the magazine before retiring to bed.  In the morning they'd  chamber the top round off the mag, remove the magazine and insert that lone cartridge into the top spot on the magazine and then insert the mag, going off to work.

When they returned after that day's shift, they'd repeat the process, essentially swapping back and forth, between the two rounds, day in and day out, week after week.

This would eventually lead to bullet setback for those two particular rounds and after a while, they'd get the random KBs.

I think this example is excessive over a period of time, but it IS evidently a problem according to Sig and Glock.

The fix was to rotate all of the ammo, from top to bottom and then after a month, relegate that batch to the practice pile, substituting fresh ammo in its place.

Chris

Almost.   I have run test and the only thing that I have done with setback hat has hurt any firearm is I
Set back a round so much I broke an extractor on a FA uzi.   The projo barely cleared the brass.

No, it's just that the manual calls for one thing, however they determine it.  Some of my 9mm reloads for my M&P9 are longer.

Everything comes to play when considering oal, everything. Most importantly, neck tension.

I threw setback into the mix because you shouldn't be scared to deviate from suggested oal, IF you follow one simple seating rule.

Seat bullet ogive, that's the curvature coming off straight wall bearing surface. Keep ogive 1/32" off case mouth.

For those who are scared of increased pressure, you're right. Deeper you seat, higher the pressure. As to setback blowing up guns, myth. Factors like insufficient neck tension, unsupported chambers, wrong powder and powder overcharges are what blow up guns.

Setback is a product of lost neck tension. Inertial sling is what becomes of setback. So be careful, leave chamber room.

Your primary concern is neck tension, secondary is feed ability and third is accuracy. Seat your pistol bullet for optimal feed ability, published oal is pertinent to weapon used for published data.

Regards,

dc.

P.S. Forgot the put the pudding into the post. Industry doesn't setback as overriding event in catastrophic case failure, cartridge related handgun failure. It's either been dismissed or viewed as a secondary or even third factor in case related failures.

One quick call to Sierra or Hodgdons  will lend perspective.

Still on topic... I discovered that I have to load XTP's fairly short (right at or below min) COL for my 9mm Kimber or the ogive starts sticking in the rifling (same cartridge does not do this in other chambers, ie: Glock). I never shot those particular cartridges (that touched the rifling) out of that particular gun (9mm 1911)...

but what you are saying is that: if I did... it most likely wouldn't be a problem - all else being equal (I subsequently lowered the charge to offset for compressed COL in the loads designed for the Kimber 9mm 1911 - getting about 1050 FPS with a 115 XTP)?

I'm going to leave your question for AeroE and paint a picture that's your real concern.

Pressure with no place to go, supported chamber and a recoil spring strong enough to hold the pressure peak.

From ignition gas moves ahead of and behind a bullet.

Chew on that for a bit.

dc.

While I fully agree with this quote, stating that no amount if setback will blow up a pistol is irresponsible.  Can you be so certain with all powders and all pistol makes?  Just because the pressure 'has someplace to go' doesnt mean the peak pressure can be easily higher than sammi specs, and potentially higher than the chamber can take.  

All that said, check the bullet manufacturers load data for OAL recommendation (as in the Hornady XTP case, OAL is significantly different from the norm), load and chamber a few with bullets seated but no powder or primer to double check the will feed in your gun, and fine tune for accuracy if you must.

OAL is determined by placing the cartridge between the jaws of a caliper and measuring the distance from the case head (the base) to the very tip of the bullet.  The measured dimension will vary a bit from cartridge to cartridge.  Small variations in measured OAL are to be expected (+/- 0.005", depends upon style of bullet used and seating stem in your die).  

Minor changes are probably not important, as long as the bullets fit the magazine, feed and extract reliably and are not jammed into the rifling.


From a reloading perspective, I think your issue is this - why does the OAL vary on some of your ammo?  Is it because you are using different bullet styles or is it different manufacturing lots of the same bullet?

You really haven't given us much to go on in helping you diagnose and fix your problem, or even understanding the true nature of your question.  Can you explain further or provide additional information?

A 180 grain bullet in a 40 S&W with .100" of setback can be over 60,000 PSI.  That's plenty of pressure to K'boom a glock.  Do bad things to a 1911 as well.


Horady did a study (using a pressure barrel).  I wish I could remember where they published this data.

The .40 S&W is the most sensitive since the case capacity is so small.  The .45 can take a lot more setback before pressures rise significantly.

Make a call for yourself to Sierra. Your question is, will setback cause my semi-auto pistol blow-up ? First, you might get a snicker. Second, tech will say setback will certainly raise pressures.

But, before you do this. You'd be well served to go back and read my posts, slowly. Take every word in because I clearly state the causes for catastrophic case failures.

Regards,


dc.

Well... I'll take a guess: perhaps it may come out the grip through the magazine? This is why I wondered how/if AeroE safely fired those (plunk test failures) into the ground...

You'd have a point if discussing gen 1 & 2 Glocks. Gens 3 & 4, no. Setback, if you believe setback to be a contributing cause of failures wasn't source of failures. Unsupported chambers were to blame for failures.

dc.

Think of pressure as a balloon being aired inside a container. The weakest point in container will be the exit point for air. Pressure follows the least path of resistance, setbacks accompany a loss in neck tension. Catastrophic failures occur with a sudden burst in pressure and exit weakest point in cartridge.

With neck tension lost, bullet presents in mechanically sound pistols less resistance than recoil spring.

dc.

eta, in Lee's example. Pressure followed the bullet because the cartridge, chamber and recoil spring withstood the pressure.

Get where we are going ?