Hello, AR15 brothers and sister:

These are the results of my tests.  I'm unsure if this would work for other calibers or platforms.  Would love to hear if anyone else has experience from extending OAL such a minor amount and experienced any increase in accuracy.

Okay, so I've been in a quest to find what my M1A likes... which was the title of a topic I posted originating March 30th.  On the second page, I posted results experienced loading different 168 gr. bullets recently with minor powder-changes using IMR 4064.

So, I got to thinking after reading a bunch of different topics floating around this website, as well as the internet:

What if I try to seat bullets just a little bit longer?  Instead of a flat 2.800" OAL - try 2.810"... maybe 2.820"... since that is the maximum length I could go and still fit in the mag.

With a bit of fluctuation in powder-charge on two of the loads for specific bullets, the accuracy changed for the better.  3 out of the 4 bullets originally tested were consistently at 1" groups or better - performing the same version of the original test.

Wish I was smart enough to test this theory on my own, but this reloading-thing is much more complex than originally anticipated.  Ever feel dumb like that?

Anyone have any idea how such a minor change in length can affect accuracy like that?  I mean, I understand that getting the bullet closer to the lands can make a difference on a bolt-action.  But, this is a semi-auto that's not necessarily known for that type of accuracy.

Anyone able to school me as to... Why?

When working up a load, I normally go 6 steps of .005". I also use the ogive to mesure from, not the OAL.

I'll give it a go.





In each bullet/powder combo, there's a certain burn rate to match the barrel's top and bottom harmonic node. Bullet depth can affect the pressure and burn rate a little bit.



That's why many advocate finding a good charge weight that's close to the top or bottom node, then refining with seating depth. One may be able to achieve the same effect with minor charge adjustments.





Some chambers have a looser freebore diameter allowing the bullet to leave the case and enter the rifling crooked. Seating closer to the lands can guide the bullet sooner with less deflection.



Some chambers have different leade angles which can affect that pressure and burn rate.
In for the experts' say.

That actually sounds logical and makes more sense, HighpowerRifleBrony.  (slaps forehead)  Not that your suggestion was irrelevant, ScooterInVegas.  (Was just focused on the OAL for magazine-dimensions, at the time.)

Being fairly new to reloading, I've been chasing accuracy with trimming brass to the thousandth, hand-weighing charges to exact weights, and seating the bullets to book / powder-manufacturer suggestions.  Never occurred to me about adjusting bullet-seating depth with a magazine-fed semi-auto... like, ever.  (duh)  Bolt-action is a completely different story.

Maybe it's the way you wrote it, or that your explanation was in the order that my mind could comprehend, HighpowerRifleBrony.

But, tuning the loads using my preconceived notions was a half-measure... in hindsight.  After finding the balance of components that worked best for my M1A, I should have kept going with the bullet seating depth to "refine" the process to complete the accuracy search.

I'm a bit disappointed since I feel that some of my other semi-auto tests aren't completed, now.

[Back to square-one.]

Depending on bullet length and construction, I load farther out until I have 2/3 of the caliber of shank in the neck or I'm within 0.010" of the magazine's limit. Between those extremes, I don't consider effective enough to warrant spending time and resources on experimenting, but I'm only looking for MOA or less.
For particularly long bullets that have to be single loaded, I find the OAL to the lands using the split case neck method, and back off 0.010 - 0.015", mostly so I don't have to worry about pulling bullets and dumping powder into my action in a ceasefire.
As always, YMMV.
ETA: They may not stay like that as long as other rifles, but this is what the M1 type rifles are capable of when all details are tended to.



*not my target

ETA2: Guess which was the first shot?
 

I'm no expert, but your experience is similar to mine. I have been working on a load for my DPMS LR-308 for some time now using good bullets and powders. I finally gave in that it was just going to be a 1.5-2 moa gun. That is until I did a seating depth test yesterday. Going shorter than the standard 2.800 OAL resulted in a .75 moa group. Finally!
I don't know what took me so long to play with the seating depth. I have found that bullets that are long or VLD style can be finicky about seating depth in my bolt rifles. I generally will start a load workup with a ladder comprised of about 10 charges shot over a chronograph to get an idea of velocity and pressure changes. I then proceed with an OCW style test followed by seating depth changes on a promising load. With bullets I think might be finicky about seating depth, I will do a seating depth OCW on a mid range load after the ladder and before the OCW. I have found that finding a seating depth that the rifle/bullet likes improves the results of the OCW.

What you've found could be concrete proof seating depth makes that much difference, or your barrel might have been optimally dirty or clean. Maybe the lighting and mirage was less or maybe you just had a better day than before.

Quite a bit of information to comprehend.  Still feeling like a dummy, though.

As mentioned earlier, I just never thought about adjusting seating depth for the semi.  (You know, the magazine and all.)  Have always done it with the bolt-actions from the start, since it seemed logical after reading and watching precision-shooters reload.

And here I thought I was getting a grip on reloading.

I'm still on the quest for accuracy.  Just have an extra variable to put into the equation.  Maybe I'll start to figure it out more by re-evaluating a couple of different bullets and go from there.  Just anxious to get one working well so I can feel something has been accomplished.

Or, maybe I need to lower my expectations a little.  Either way, there's work to be done.

Thanks everyone.  Always appreciate the many answers and suggestions by the members.  Glad to be here.

I first determine the length the rounds will be made, then find the load, there are other things that will help with your quest for your best load
 I use an RCBS PRECISION MIC set
 first to determin the distance to the lands ( differant bullets work best at differant distances off the lands )
 next to determin the size of the chamber ( from a few fired cases )
 next to help me set the sizing die (I size 2 thou. smaller than the chamber )
 and last to measure the completed round ( to the point where it touches the lands )

 I have found that the bullets I use ( both Sierra and Horrnady A max and Hornady SSTs work best between 10 to 15 thou. to the lands )
 at that length they do not fit in the magazine well, so for the rounds to be used in the magazine I reduced the COAL to 2.800
 ( that fits all my 308s ) ( Rem 700, Israeli Mauser, 3 rebarreled Mausers, Indian enfield )

 The completed rounds were made for a REM 700 and work very well in a friends M1A1
 they both use the same hole at 200 yrards with the longer than magazine length rounds
 they both  at 2.800 (mag. length for my 308s ) have the holes touching at 200 yards
 That slightly larger group size is acceptable for rounds in the magazine and the other measurements of the cases
 help with reducing the group sizes
 the load for both is the same (42.2gr IMR 4064  ( at or near sea level, the load is reduced .1gr at 2000 ft above sea level ))

 I chose the RCBS MIC. set because it has all the measuring tools needed in it

 The REM has a chamber 1/2 thou. small, the M1A1 has a chamber 1 thou. over 0
 In these completion rounds, all cases are weighed, all bullets are weighed, bullets bearing surface is measured
 powder charge is weighed twice  ( they are used at 600 yards )

When you change the seating depth you change a number of things, eg, amount of case bearing on the bullet, case volume and initial pressure, etc. In most cases the change is small but results in a change in velocity which changes the amount of time the bullet spends in the barrel (BT). as mentioned by others,

IMO the most likely cause of your accuracy improvement was getting your ammo's average BT closer to the center of the barrel's harmonic sweet spot. Now more of them fire like the previous one despite unavoidable small changes in round-to-round velocity.

0.020" off the lands is usually a pretty good default for the traditional bullet profiles.  I usually start there when I can (I have a pretty tight match chamber and can usually do that and still fit in the mag).  With typical factory chambers, I go as far as i can within mag length while keeping roughly 1 bullet dia in the case neck.

A rifle's throat and the distance from the ogive to the leade (where your rifling starts) is one of the variables for optimum accuracy.

Figure a rifle throat will erode .001 inch per thousand rounds fired.

M118 Long Range and the SOCOM Mark 316 are seated to 2.830", give or take .030 inches -- easily within mag length for the M14 or AR-10.

Have you actually proved this yet. Like Kendoc posted there are many factors that effect accuracy. Go and repeat the results time and time again before you claim victory.

I'm in no way suggesting that it doesn't have an effect. I'm just sayin don't feel bad if the next time out it's not the same.

Motor

Thanks so much for the large amount of information, firewrench044.  Proverbially re-wiring my brain to take the extra steps needed to increase accuracy on this particular platform.  It's a work-in-progress, but I'll get it.

A minor goal is to find a suitable-load that actually fits in the mag, and can produce consistent accuracy.  For now, I realize that will probably end up like trying to kick water uphill... but, I'm willing to try and learn more from this experience.

That's the trick isn't it, Twoboxer?  Finding that elusive sweet-spot.  I'm hoping that I can try a few variables with powder / bullet-seating this weekend to get a little closer to my goal.  Hoping it doesn't rain.

I've loaded rounds in .1 increments to .5 grains both up and down from the powder sweet-spot seating at .010, .020, and .030 for each variation.  (Of the best performing bullet, for now.)  Will try and see what results from tests and report back.

I've marked up the bullet with a Sharpie and closed the bolt without the spring to find the lands, 30Caliber.  (Adjusting seating depth each try.  Same way I've done the bolt-actions.)  May take firewrench044's advice and invest in the RCBS Precision Mic Set.  Still have to get the Hornady set of headspace gauges that dryflash3 suggested, as well.  (Making notation on my to-do list.)  The only fight I have on my hands beyond that is getting the loaded rounds in the mag.  And, there's the "give-and-take".

Will continue to "tinker" with what I find for repeatability, Sinister and Motor1.  Have dedicated an amount of time and thoroughness to develop this load due to enjoying the platform so much.  Just taking every possible avenue of discovery to find the potential of what I have here.  Further testing is mandatory and the plan is finding the combination that works consistently.

I know that my expectations are a bit "lofty" on this, but the least I can do is my part.  Fortunately, I've learned that seating-depth on one test has produced positive results.  Soon, I hope to confirm that and possibly find an optimal combination using these components.  It's a process.

Thanks to everyone for contributing to this conversation.  I've learned so much from this forum and the many folks here.

Will post results - hopefully with pictures - very soon.

Skinnable. Kind words indeed.

We all learn from these threads. One of my most recent load work ups was/is for my son's M1A Loaded. We have found the sweet spot to be 40gr of Varget in LC-63 brass.

After reading your thread I am very tempted to go back a mess with seat depth a little just to see the effect.

Currently we are using 2.800" OAL which leaves room for a little experimenting. Oh. The bullet is a Hornady 150gr SST

Motor

 The RCBS PRECISION MIC does the same thing as the Hornady gage  and more so
you do not need both

Unfortunately, rain held off my day of tests.  Oh well.  That gives me more mosquitoes to contend with, I guess.  Hooray !!!

Hope to figure this stuff out relatively quick, Motor1.  A buddy mentioned that his new AR-10 liked the Hornady 168 gr. A-MAX to be seated 2.780"... short, like Kandoc mentioned in his post previously.  But, when my buddy loads Sierra 168 gr BTHPs, his AR likes 'em to be seated at 2.810".  Minor powder changes are also mentioned... but, not by much.

Was planning to purchase the RCBS Precision Mic Set for the .308, firewrench044.  The Hornady Headspace Gauge Set covers more calibers and could fill a void for other loading processes.  Couldn't justify dropping $50 for each caliber I'm loading on the RCBS-stuff, now.  Things like, powder, primers, and bullets have been the focus lately.

Will post my results soon.