Does seating depth a have this large impact on group size?

[Bigeclipse]

All,
just a quick question, can seating depth change a group from say .5inch at 100 yards to 1.5inches or is something else likely going on? Here is what happened (long read but please read all). I put together a rifle for my wife using a savage action and after market barrel. Shot it with some loads of accubonds and varget powder at .010 off lands. Shot ok (~1inch group). Then loaded up two different powder charges at .020 off the lands. These charges were 41.5 grains and 42grains. The 41.5 charge group came in at .75inches and the 42 grain group came in at .5inches. I then said to myself ok...wife will only be shooting to 300 yards so don't really need to do any real load development this year and will call the 42 grain load with .020 off lands good. I loaded up 10 more of these. Went to the range and shot with her and the groups were not good at all...were right around 1.5inches. Now here is the deal, went home confused...went to load up some more and realized I never zeroed my calipers and the loads were potentially seated at .040 inches off the lands instead of .02., I had left the calipers on and noticed this when I got home (I know VERY stupid mistake). Now that being said, could this change in seating depth really cause that drastic change in group size or is there likely something else going on like maybe a loose scope? I do realize that changing seating depth impacts group sizes but the thought that it impacts it this much is making me scratch my head. Everything else was the same, same primer lot, bullet lot, powder lot, same neck tension and brass and brass prep...etc.

Full disclosure: I did have an issue with the rifle not making good groups with factory loads until I realized the front scope mount screw was touching the barrel so it was not tightening down all the way...I filed it down until it no longer touched, screwed everything down with Loctite (mounts and rings). This was when I went and shot the rifle with the loads being .020 off the lands and getting the .5-.75 inch groups. This is why im thinking maybe there is more to my issue than the change in seating depth and maybe it is the scope mounts but dang, I screwed them down snug with loktite and they shot fine prior to this.

I will be checking everything over tonight and reloading some more ammo but just wanted to know if seating depth alone "COULD" potentially do this? If anyone has witnessed seating depth ruin a load this badly please let me know? Thanks!

 

[rcoody]

All,
just a quick question, can seating depth change a group from say .5inch at 100 yards to 1.5inches or is something else likely going on? Here is what happened (long read but please read all). I put together a rifle for my wife using a savage action and after market barrel. Shot it with some loads of accubonds and varget powder at .010 off lands. Shot ok (~1inch group). Then loaded up two different powder charges at .020 off the lands. These charges were 41.5 grains and 42grains. The 41.5 charge group came in at .75inches and the 42 grain group came in at .5inches. I then said to myself ok...wife will only be shooting to 300 yards so don't really need to do any real load development this year and will call the 42 grain load with .020 off lands good. I loaded up 10 more of these. Went to the range and shot with her and the groups were not good at all...were right around 1.5inches. Now here is the deal, went home confused...went to load up some more and realized I never zeroed my calipers and the loads were potentially seated at .040 inches off the lands instead of .02., I had left the calipers on and noticed this when I got home (I know VERY stupid mistake). Now that being said, could this change in seating depth really cause that drastic change in group size or is there likely something else going on like maybe a loose scope? I do realize that changing seating depth impacts group sizes but the thought that it impacts it this much is making me scratch my head. Everything else was the same, same primer lot, bullet lot, powder lot, same neck tension and brass and brass prep...etc.

Full disclosure: I did have an issue with the rifle not making good groups with factory loads until I realized the front scope mount screw was touching the barrel so it was not tightening down all the way...I filed it down until it no longer touched, screwed everything down with Loctite (mounts and rings). This was when I went and shot the rifle with the loads being .020 off the lands and getting the .5-.75 inch groups. This is why im thinking maybe there is more to my issue than the change in seating depth and maybe it is the scope mounts but dang, I screwed them down snug with loktite and they shot fine prior to this.

I will be checking everything over tonight and reloading some more ammo but just wanted to know if seating depth alone "COULD" potentially do this? If anyone has witnessed seating depth ruin a load this badly please let me know? Thanks!

always remember change only one thing at a time

yes seating depth can certainly change a group like that. Some bullets are more forgiving than others. Remember as you seat a bullet deeper you are removing case capacity. This can affect velocities and pressure. It also affects barrel harmonics along with the jump to the rifling and how that bullet hits the rifling. finding that sweet spot of powder charge and seating depth is better done in a very organized manner. It will make a huge difference when you find it.

 

[Barrelnut]

I'm not clear on who shot the second round of 10 with the .040 jump. You or your wife?

It's only a .020 change with a 1" change in group size. That does sound excessive. Berger's write up mentions that seating sweat spots can be around .030 wide. So, you would be within that. I shoot a lot of Accubonds in a 300 WSM and 25-06. I load both of them with a .010 jump. Cannot remember a .020 change in jump causing that large a difference in group size. But there is always that disclaimer "every rifle is different".

 

[Bigeclipse]

always remember change only one thing at a time

yes seating depth can certainly change a group like that. Some bullets are more forgiving than others. Remember as you seat a bullet deeper you are removing case capacity. This can affect velocities and pressure. It also affects barrel harmonics along with the jump to the rifling and how that bullet hits the rifling. finding that sweet spot of powder charge and seating depth is better done in a very organized manner. It will make a huge difference when you find it.

Normally I would do a full load development (ladder testing...etc) but since my wife will only be shooting to 300 yards and more likely under 150 yards and I did not have time to do a full work up I decided to roll the dice and try to find a quick load around 1MOA. once I sorted the original issue out of the scope base screw touching the barrel, I then did an initial pressure test to book max at .010 off the lands to make sure safety. Then tested the book max load at .010 off the lands to see what the barrel did. This load gave me ~1inch group at 100 yards. I then did book max and .5 grain less at .020 off lands just to see what happens. The groups shrunk to half inch and .75 inches. This was great and I simply called it good. I was shocked to see such an awful group this weekend. The first thing that came to mind was scope mounts until I found my calipers were not zeroed. I was still suspicious of my scope mounts since the group was sooo awful but wanted to see if anyone else has seen a group this bad solely to seating depth change.

 

[Bigeclipse]

I'm not clear on who shot the second round of 10 with the .040 jump. You or your wife?

It's only a .020 change with a 1" change in group size. That does sound excessive. Berger's write up mentions that seating sweat spots can be around .030 wide. So, you would be within that. I shoot a lot of Accubonds in a 300 WSM and 25-06. I load both of them with a .010 jump. Cannot remember a .020 change in jump causing that large a difference in group size. But there is always that disclaimer "every rifle is different".

we both did shooting. She shot first 5 and was getting a 1.5inch group. I thought it might be her so I shot the next 5 and also got about a 1.5inch group.

I was also thinking like you that the group size changed so drastically it had to be something more than seating depth. There is one more thing, the groups looked like a pretty standard group with no random flyers. Whenever I had a scope issue, it usually showed flyers...so maybe it truly is the seating depth. We shall see tomorrow.

now as far as your comment on berger saying .030 wide. .010 shot ok, .020 shot even better...well .040 shot terrible...maybe .010-.030 is the "sweet spot" and suddenly .040 is now that "scatter" node when speaking about the OCW test...I really cant be sure with out any load development.

 

[Bart B]

Keep in mind that several 5-shot groups fired with the same load in the same rifle at the same range can easily have a 3X spread in extreme spread; sometimes more. What your're seeing is typical of shooting a group or two at short range then another one or two more at a longer range.

Most barrels won't show a significant accuracy change over a .050 inch change in bullet jump distance to the lands with a good load; if you test with at least 15 shots per test group..

 

[engineer40]

The biggest improvement I myself had got is going from about 1.25 inch 5 shot groups to .25 inch groups messing with just the seating depth. So it is certainly feasible.

I usually will keep some cheap factory ammo around like Prvi. I keep saved targets. When Im confused with my reloading results and I start to question if it is the rifle or my reloading, Ill do a group with my "known quantity". It doesnt matter if the factory ammo doesnt shoot great groups. But if that stays consistent, well then most likely its my reloads. Hope that makes sense.

I once had a rifle that was shooting crazy. Good groups sometimes then huge groups. I couldnt figure it out. Im positive I shot over 100 rounds trying to determine what the problem was. I started to think the dang barrel might be coming off the receiver or something. The scope felt tight. It didnt seem to move and it did not make any noise when i tried to wiggle it. But i did remember the scope base had come lose the summer before. Then I also remembered I mounted that scope back when I just trusted that Loctite always worked. So I took the scope rings and base off, cleaned all male and female threads with Non-Residue brake cleaner, put it all back together (again using Loctite) and the rifle shot great again. Almost every metal part in the firearms industry gets shipped with a thin layer of oil on it. Loctite wont do its job if there is any residue at all on the metal. Since then both my Dad and an Uncle had the same exact problem and their scopes were installed by a gunsmith. The permanent fix for both was the same as I did above.

Hope this helps.

 

[Bart B]

Remember as you seat a bullet deeper you are removing case capacity. This can affect velocities and pressure. It also affects barrel harmonics...

Barrel harmonic frequencies, as well as the barrel's resonant frequency, is the same for every shot as long as it's totally free floating. The barrel's metal properties don't change from shot to shot and neither does the frequencies it vibrates at. The amplitude the barrel whips and wiggles at will be the same for each shot of a given load.

The amplitude and directions of those vibrations are primarily controlled by the direction of and how far the bore axis is away from the center of mass holding the barrel. These are controlled by the stock design and how the rifle's held by the shooter.

If the barrel bounces off any point in the stock fore end, the harmonic frequency that controls the bullet exit direction at the muzzle will change with the external pressure applied to the fore end.

 

[Rich Coyle]

I once did a test at 200 yards with a known load. I fired ten shots with the rifle laying on the bags loosely and me trying not to influence it. The group measured .870". Then I did the same thing with me holding it on the bags but like I might in a varmint shooting setup. The ten shot group was 2"!

I don't know what this tells you about your "problem".

 

[rcoody]

seating depth can have a profound effect on group size. I had been looking for a mag length load for my .308 using the berger hunting vld. I read the berger jump article and tried it. After finding my jump I decided to try fine tuning it. The target below shows a series of 5 shot groups seated 0.01 deeper each time. you can easily see the first group is out of the node. Berger says the node is 0.03 wide and this shows that clearly. then the node goes away.

 

[Bart B]

rcoody, I don't think 3-shot groups are statistically significant to have much meaning for your test.

I've shot many, many 3-shot groups totalling up to a 30-shot composite. They typically had a 4X spread between the largest and smallest one. And the first one fired was not always the smallest. The 30-shot composite was larger than the biggest 3-shot group.

 

[Rich Coyle]

rcoody, I don't think 3-shot groups are statistically significant to have much meaning for your test.

I've shot many, many 3-shot groups totalling up to a 30-shot composite. They typically had a 4X spread between the largest and smallest one. And the first one fired was not always the smallest. The 30-shot composite was larger than the biggest 3-shot group.

This is hard for most folks to accept. I no longer stop at three shots unless I have a know shooter rifle. It would do ten shots in .4. What was a surprise it was an out of the box plastic stock Savage .223 with a 26" 9 twist barrel.

 

[Bart B]

In response to 3-shot group's statistica insignificance...

This is hard for most folks to accept. I no longer stop at three shots unless I have a know shooter rifle. It would do ten shots in .4. What was a surprise it was an out of the box plastic stock Savage .223 with a 26" 9 twist barrel.

There are other shooting/reloading facts that are hard for most people to accept and impossible for a few.

 

[rcoody]

rcoody, I don't think 3-shot groups are statistically significant to have much meaning for your test.

I've shot many, many 3-shot groups totalling up to a 30-shot composite. They typically had a 4X spread between the largest and smallest one. And the first one fired was not always the smallest. The 30-shot composite was larger than the biggest 3-shot group.

so for you the target I shot and the results I see are just a statistical anomaly.
reality is those groups are all 2" or greater.

opinions vary!