Seatting Depth
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Old 05-19-2012, 12:40 AM   #1
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Default Seatting Depth

I tried some different loads out in my .308 yesterday. The first five were light on powder compared to what I normally shoot and the second five were a little hot. I really didn't see any change in groups between the two. Both were a little worse than I normally shoot and were about 1.23" at 100 yards. Next I tried five shots .015" longer than I normally shoot and then I tried 5 shots .015" longer than that, or .030" longer than my normal load. Powder charge was what I normally shoot which is in between the light and hot loads I tried to start with. The .030" longer bullet fired the same size group as the first two loads. The .015" longer was a different story. I had 4 shots in one whole. I saved the 5th round just to make sure I wrote the length down right!!! That then brought up some questions.

I was really surprised to see the my group size change that dramatically with a .015" change in seating depth. So I'm wondering how many of you have seen that sort of change in group size from such a small change in seating depth?

Second, I realize that just because I got one great group from four bullets doesn't mean a whole lot. That could have been just luck. So what are some different thoughts on how many shots should be fired when checking a load?

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Old 05-19-2012, 02:35 AM   #2
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I know long distance shooters actually change the lengths according to their rifles chamber leaving as little of head space as possible so there its literally no space for the bullet to travel before entering the rifling of the barrel..but I literally just started reloading so I have no actual experience..this is just information ive read on other blogs

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Old 05-19-2012, 11:27 AM   #3
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I have a couple rifles that are very sensitive to OAL and a .015 could make a big difference. The rest of my rifles are much less sensitive and an OAL change of .015 would do little if anything as far as accuracy goes.

Load up 10 more each with both OALs and test again. Shoot five of one then five of the other and see if the improvement continues. It would be great if you could have someone else load the rifle for you so you did not know which length you were shooting.

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Old 05-19-2012, 12:04 PM   #4
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Make sure you fire 2 or 3 fouling shots from a clean bore, I usually don't get good groups till then but it is always important to know where the 1st shot lands. I was loading for my 223 target mini and I have great load for it with 55 gr Nosler and 21.5 gr RL7 col 2.260. The same powder and col in the Hornady fmj is terrible, I went back to what Hornady recomended at 2.220 and it shot great.

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Old 05-19-2012, 01:02 PM   #5
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you said .308 but not what kind of rifle???

for bolt rifles, try to figure out what length your bullet just kisses the lands and start there. make dummy rounds for testing: sized case with bullet, no powder, no primer. chamber the dummy round and then eject, look close with a magnifying glass to see if the lands left marks on the bullet. Not touching: no marks. Jammed hard: rectangular marks. Just right: the marks will be almost perfect squares. take notes on what is jammed, not touching, etc. Start out just touching and work up a powder load. (you seem to already have one that works well so use it) Only change one thing at a time, shoot 5 shot groups, 2-3 foulers before starting the first group, and clean after every 2 groups (10-13 shots) .015 is a pretty substantial change. you may wanna make changes in increments of .002-.005 at a time. It all depends on your definition of accuracy and what you're looking to achieve.

you will initially see groups start to shrink with the changes and then eventually open back up. Refer to your notes to go back to where they were good and fine tune. Keep in mind that the throat will erode with use and lands move further away from the chamber due to wear. Eventually you will have to increase OAL to chase the lands and that "sweet spot"

For semi auto's, I just load to the max OAL that will function in the magazine and go from there. But I also don't require any precision from my semi's. My bolt rifles are reserved for precision shots.

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Old 05-19-2012, 02:47 PM   #6
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Set depth by the manual and you're good to go. I think that far too many of us obsess about seating depth.

OTOH, if you shoot benchrest, buy a Sinclair measuring tool and bullet comparator.

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Old 05-19-2012, 03:46 PM   #7
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"The .030" longer bullet fired the same size group as the first two loads."

I assume you meant to say "cartridge", not "bullet"? You seem to have proven that 15 thou can make a difference. But you know you'll need to repeat it a few times before you start to count on it; statisically, a single example of anything doesn't mean a lot. I often fire 20 rounds of what I really want to prove and use the results of that. Averaging a series of groups gives us a way to rationalise the occasional 'flyers' and pretend they don't matter but we shoot them too, don't we?

If I limited my reloads to what's "in the book" for either powder or OAL, I'd quit kidding myself about custom tayloring my ammo to my rifles and just shoot factory stuff.

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Old 05-19-2012, 03:48 PM   #8
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I have nothing to add to this conversation...other than to say i am reading intently trying to learn.

If your dropping 4 shots in the same hole @100 i would say its skill and not luck.

Is the gun locked down in a "lead sled" or are you "free handing"?

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Old 05-19-2012, 07:23 PM   #9
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I have both a SA NM M1A and Savage 10FLP that shoots .308. I was experimenting with different bullet seating depths for ammo I was shooting through my Savage. I used my Hornady bullet seating measuring tool. I wasn't getting the results I had hoped for. I tried some loads I had developed for my M1A through it and was quite surprised at how well they grouped. I have two loads that will cloverleaf four shots at 100 yards every time. These cartridges had an O.A.L. of 2.80 because that is the maximum for smooth feeding from the magazine for the M1A. Now I seat all of my .308 loads to 2.80 and get great results from both rifles.

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Old 05-19-2012, 10:28 PM   #10
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Quote:
Originally Posted by ryguy00 View Post

you will initially see groups start to shrink with the changes and then eventually open back up. Refer to your notes to go back to where they were good and fine tune. Keep in mind that the throat will erode with use and lands move further away from the chamber due to wear. Eventually you will have to increase OAL to chase the lands and that "sweet spot"
I'm shooting a bolt action (Remington 5R), and what you described is what I was expecting to see. I was expecting to see the groups either open or close as I got closer to or further away from the right seating depth. I guess I just got lucky and the load that was .015" longer was the perfect load?

As for how I got to that point, I used a dummy round to find where the groves started. The groves in this rifle were way too deep and my OAL length where it made contact was at approximately 2.910". I then backed off .050" from that as a safety factor. A cartridge with the bullet touching when it goes off will increase pressure. Bench rest guys want the bullet right at touching but they are developing their loads for that and backing off on the charge to compensate for the increased pressure. So I backed off .050" from touching and then started working my way to that point stopping before I get too close. My longest loads were still .020" from touching.

In this case I'm way past the manual on OAL. I just didn't expect that much of a change from a .015" change in depth? I guess .015" really is a pretty big change in seating depth.

FMJ, I was shooting off a bench with a bi-pod and bag at the rear of the stock. I personally think most of use are capable of shooting great groups but our equipment is the biggest limiting factor? But I've also been shooting a long time and shot competitively on the rifle team in high school. And this still could have been pure luck? I will definitely load some more rounds at this length to verify my results.

I'm just trying to figure out how many rounds to load when testing a load. I don't want to load too many because at some point I'm wasting shots and time. In this instance I was trying 4 different loads with 5 rounds each. That's 20 rounds and could have been a lot more if I loaded more rounds for each load. At the same time I want enough shots to have confidence in my results. One 3 shot group could be misleading because you could get lucky and shot one good group, or one bad one. 10 shots per load would give pretty reliable results but that's a lot of shooting and time. In this case that would have been 40 rounds. So I'm looking for the bare minimum shots needed to have some confidence in the group. Right now five seems about right for me?

As for cleaning, I clean every 20 rounds as a general rule which is in part why I loaded 20 rounds to try on that trip. After I verify that I have found a great load for this gun I'll shoot more rounds and figure out at what point my groups start opening up or POI changes. I'll then change my cleaning to match what I need for this gun.
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