bullet seating depth

[nnn66]

I recently read an article on bullet seating depth and then decided to perform the test with my rifle. I'll attempt to post a link to the article here:

Optimizing Precision And Accuracy From VLD Bullets

Anyway, I loaded up 4 different sets of rounds with bullets seated at different depths starting at .010 off the lands and progressing in increments of .040 all the way out to .130 off the lands. According to the article, I would find a sweet spot near one of these lengths. I performed the test this morning with my CZ 550 Magnum HET 338 Lapua. I have only been working with this rifle for a short time and have a total of 85 rounds through it so far. I was using 300gr Lapua Scenar bullets, Lapua brass, Federal Match Magnum primers and 91.0gr of Retumbo, shooting at 100 yards. I was shooting off of a bipod in about a 12mph cross wind.

I deviated from the author's suggestion of shooting two 3 round groups of each length and, instead shot a single 5 round group of each length.

My results were as follows: At .010 off the lands, my 5 round group was 1.8 inches. At .050 off the lands, my group was .610 inches. At .090 off the lands, my group was again .610 inches and at .130 off the lands, my group was 1.5 inches. I know...nothing very impressive there, but it seems that groups were noticeably tighter at .050 and .090 off the lands. I'm thinking that the sweet spot must be somewhere in between these two depths.

This is my first attempt at playing around with seating depth, so I'm not quite sure how to interpret these results. I'm hoping that those of you with a lot more knowledge on how seating depth can affect accuracy might chime in and give me some feedback on the subject. Any suggestions on where to take the testing from here? Should I start experimenting with increments between .050 and .090? Thanks for any help you guys can provide!

 

[Dan Newberry]

At 91 grains of Retumbo with the 300's in Lapua brass, you're very close to a scatter node, which would explain some of the groups measuring over MOA.

The seating depth is something you would "fine tune" the load with after identifying the Optimal Charge Weight of powder. An OCW load will not be seating depth sensitive, unless you're using bullets which have a severe sensitivity to seating depth (which I would never recommend)...

Try 89.3 grains of the Retumbo with the 300's and choose whatever depth you like... Magazine length or 20 thousandths off the lands, whichever is shorter, would be a good place to start.

I think if you'll take time to try the 89.3 grain Retumbo charge and report back, we'll find that you have a very accurate load that will not be all that sensitive to seating depth.

Dan

 

[nnn66]

Thanks for your reply, Dan. I actually increased my charge of Retumbo to 91.0 this morning because I thought I was having signs of low pressure the last time I shot with 89.0 grains. With 89.0, my case necks were covered in dark black soot. Same thing happened at 87.5 grains. This morning, with 91.0gr, I still had some black soot on the neck, but there was not nearly as much of it as before. I have only been shooting brand new, unfired Lapua brass that has never seen a sizing die. Also, at 89gr, it seems that my neck expansion was lower than it should be as I measured neck diameter with a micrometer and they had not expanded as much as others I had shot with N570 and Magpro. Therefore, my reason for jumping up to 91 grains was to see if I could get past these signs of low pressure. However, I'm certainly willing to drop back down to 89.3 and give it a try. I had some fairly decent accuracy results at 89; I just didn't like all that black stuff on my necks.

 

[nnn66]

I need to update what I had written in my last post to this thread. I had said that the cases I fired this morning with 91gr of Retumbo had less black soot on them than the ones I had fired using 89gr Retumbo. Also, I stated that I had measured the case neck diameter and that they had not expanded as much as those fired with other powders. I went back and more closely inspected these cases and those statements are not true. I think there is about the same about of soot on them and the case necks are mostly the same diameter. I must have done a poor job of measuring last time because they are all pretty close to the same.

Dan: I did some reading on OCW and I saw that you had stated that you usually start off about 5% below max charge weight and then work up from there. If that is so, I'm wondering why 89.3 is the weight you have suggested for Retumbo with 300gr Scenar as that is exactly 5% below max of 94. Is that something that you have determined through your own testing...that going closer to 94 would not yield better results? Should I start at 89.3 and continue working up toward 94 or is there a reason why I would not wish to do so?

That black soot on the case necks still has me concerned and I'm still wondering if it might be due to low pressure. It seems unlikely that 91gr of Retumbo would produce low pressure, doesn't it? When I inspect cases that I fired using Magpro and N570, there is absolutely no soot on the case necks, but every one of the cases fired with Retumbo has heavy soot on the necks. Perhaps my gun just doesn't like Retumbo?? I hope not because I just bought 6 pounds of it!

 

[High Binder]

I'm wondering why 89.3 is the weight you have suggested for Retumbo with 300gr Scenar

FYI, 89.3grs of Retumbo seems to work well in a lot of 338LMs. I personally use H1000 (because I have a lot of it) but my 338 likes 89.3gr Retumbo a lot too.

 

[nnn66]

It will be about a week before I have another chance to shoot, but I plan to try 89.3 and will certainly report back here with results. Thanks!

 

[Dan Newberry]

We identified 89.3 grains of Retumbo as an OCW load a couple years back, and I published that load recipe on my OCW site. Many folks have found itis a good, stable, working load that doesn't burn the candle at both ends.

In my 338LM, the chamber is tighter than perhaps it has a right to be. But it's easy on brass, and the 89.3 grain Retumbo load gives me--and another Savage 338LM shooter I work with--exactly 2700 fps from Sierra 300 SMK's.

There was another node at 92 grains. Then one above that at around 94.5 grains or very close to that.

At 3 dollars a case for brass, I chose to use the 89.3 grain charge because it gets the job done--even at a mile, as we've used it at that distance.

What is the make of your rifle? Depending on the chamber, it might serve you well to move up to 92 grains and give that a try.

Dan

 

[nnn66]

Dan,
My rifle is a CZ550 Magnum HET, built by the CZ custom shop on the CZ550 Safari Magnum action. I will plan to try 89.3 this weekend and I may try some at 92 as well. I will definitely report back here with results. Thanks for the help!

 

[Dan Newberry]

Carbon fouling on the case necks isn't necessarily a bad thing, but it is always a good idea to brush the chamber out well with a good solvent, to keep carbon from caking in the neck area. This can eventually cause a "tight neck" chamber situation where the bullet doesn't want to release easily enough, and can drive pressure up pretty high. So be sure to keep that chamber nice and clean, and always brush the neck area well with a good powder solvent...

Dan

 

[nnn66]

I finally got a chance to shoot a little bit this morning and tried two different loads of Retumbo with 300gr Scenars. I shot a 5 round group with 89.3 grains and a 5 shot group with 92.0 grains. I alternated between targets as per round robin fashion. I was shooting off of a bipod at 100 yards and there was a slight wind. I know that I pulled at least one round on the 92 grain target. My group was right at 1 inch with 92 grains, including my pulled shot, and it was .650 with 89.3. My group with 89.3 had 3 bullets through one hole and then there were 2 shots through the same hole about 1/4 inch away from that hole. It certainly could have been me since I wasn't feeling real stable on the bipod this morning. All of my shooting through this rifle so far has been with virgin Lapua brass. I now have a bunch of once fired brass, so I intend to neck size and try this same test again tomorrow with my fire formed brass. I'll keep you guys posted on the results.

 

[jephthai]

I just did a seating depth test as well, and found that 0.040 was best. These are 190gr vlds in my 308. I thought that was weird, since I had expected closer to be better. Then I found a few articles from berger and others and discovered that 0.040 is a common sweet spot. I felt a little more validated then.

Sent from my ASUS Transformer Pad TF300T using Tapatalk 2

 

[nnn66]

I tried again this morning with 5 shot groups. I used 89.3, 89.6 and 89.9. I'm really getting frustrated as I will get 3 rounds through the same hole and then another 2 rounds through the same hole except the 2 holes are 3/4 inch apart! I'm using the round robin method and allowing my barrel to cool sufficiently, I think. My cases are identical, seating depth is identical, powder charges are meticulously weighed....I would guess that it has something to do with the barrel heating up except that it is not always consecutive shots through the same hole. What else do I need to be considering?

 

[doc76251]

I tried again this morning with 5 shot groups. I used 89.3, 89.6 and 89.9. I'm really getting frustrated as I will get 3 rounds through the same hole and then another 2 rounds through the same hole except the 2 holes are 3/4 inch apart! I'm using the round robin method and allowing my barrel to cool sufficiently, I think. My cases are identical, seating depth is identical, powder charges are meticulously weighed....I would guess that it has something to do with the barrel heating up except that it is not always consecutive shots through the same hole. What else do I need to be considering?

You are experiencing split groups. "Generally" it comes from losing position in the middle of the string. Whether it be from attempting to stay in one position or more likely in the case of a 338, not noticing that your position has changed. When you do toss the shot you think "%^#$" and you get back on the gun and the bullet goes through the first hole. Then it slides a little and you're right back where you were when you tossed it the first time and it goes in the second hole. Try rebuilding your position after each shot. That is, get up walk around, line up and get in it again. Oh one other thing, when shooting for groups NEVER EVER shoot at what your aiming at.

Food for though.

Cheers,

Doc

 

[nnn66]

Thanks for the reply, Doc. I have had the local range to myself the past two mornings. I guess nobody gets out of bed as early as I do. Therefore, between shots, I have been walking around and picking up 223 brass that is lying all over the ground. It gives my barrel a chance to cool, lets me take my mind off the target for a moment and it helps fill my bucket of 223 brass! When I do get back to the rifle, I repeat everything and set up as before. I think I am consistent. And, of course, my point of aim is always the same. I'm going to try again tomorrow morning and see if I get similar results. I don't think it is me because I've gone out there with a buddy and shot 1/4 moa 5 shots groups with his 300RUM and then turn around and shoot a 1 moa group with mine. In fact, I let him shoot mine and the results were similar.

 

[doc76251]

Well it was a thought. Is that action bedded per chance or riding on an aluminum block? I had a similar experience with my Winny HBV until I started messing with torque settings when it was in the original HS-P stock, it was resolved completely when I bedded the action to the block.

Cheers,

Doc

 

[rcamuglia]

Hey nnn,

You may want to try a different scope on your rifle. They do funny things when they go bad and double grouping is one of them. It won't take long to find out

Dan,

I'm curious as to how you have found 89.3 grains to be some kind of "universal" load in the .338 LM? Powder lots vary, chambers vary, and even bullet lots do. For example, my last lot of H-4350 was 41.3 grains with the 140 A-Max in the 6.5 Creedmoor and my new lot of H-4350 load is 43.6 grains.

Standing by.

 

[nnn66]

Doc,
Here's partial description of the rifle from CZ:

The H.E.T. model is equipped with a Bell & Carlson tactical Kevlar reinforced stock with a full length aluminum bedding block, a 28" barrel and a SureFire muzzle break.

I'm assuming this is what you mean about riding on an aluminum block. Would I benefit by having the action bedded to the block? I don't have any experience with bedding the action to a block, so I'd have to find somebody who does this and have it done, I suppose. If it's something that is going to help this rifle to shoot like it should, then I would definitely want to have it done. I guess I need to learn more about that.

rcamuglia: I suppose it might be the scope, but I'm guessing not. I'm certainly going to keep searching until I figure out what the problem is. I was wondering the same thing about Dan's "universal" loads, but I went to his website and did some reading which pretty well explained it. I think there are a couple of links to another guy's work that you'll have to read in order to get the entire gist of it. I'm sure they are a helluva lot more knowledgeable than I am on the subject, so I'm taking their word that it works. However, I'm hard to convince until I've seen it for myself, so I'm trying to find some magic in that 89.3 load. I haven't given up on it yet!

 

[doc76251]

Yep, not HS but same principle and truth be told I think B&C does a better job with their blocks. Before looking for a bedding smith I'd get a torque wrench and pull the action. With just an allen wrench I'd put the screws in one at a time and see if (visually) the action lifts on either end as you tighten things down on one screw. If you tighten down the rear/front and it looks like the rifle is trying to climb out of the stock you found your issue, the receiver is not supported on one end or the other. If all goes flush and the bolts up snug, flat and level then play with the torque settings. In general the front is torqued first and may be a bit heavier. Just stuff I tried and found some success with before I said screw it and did my first bedding job.

Cheers,

Doc

 

[nnn66]

Thanks Doc. I'm think I'm going to do a little more shooting in the next day or two and see how it goes. Then, if things are still not coming together, I intend to take your advice on this. Where would I find specs on how much in/lb torque to apply to the action screws? I suppose I could call the guys at the CZ custom shop who built it and ask them, but just wondering if there is a general rule on this or are all action/stock combinations different?

 

[gasdoc09]

Oh one other thing, when shooting for groups NEVER EVER shoot at what your aiming at.

What does this mean?

 

[doc76251]

Short version: Utilizing the concept of aim small, miss small you pick a very precise point on the target, say the intersection of two lines. If your rifle is dialed in and you take a shot and the bullet hits that aiming point but not perfectly you no longer have the same spot you were aiming at. So what will you do? Use the center of the bullet hole or where you "think" you were aiming, not precise. Dial up/down a MOA/Mil or two so that your impacts are off your aiming point, it will still be a group you just selectively moved it out of the way a measured distance. For doing OCW I even made targets with the aiming point 3" from my "bullseye".

Cheers,

Doc

 

[nnn66]

After doing some research, it seems that split groups, like I've been experiencing, are a common indication that the action needs to be properly bedded, so I have made a decision to have this done. I figured there's not much sense in continuing to try working up loads until I know that there isn't another problem skewing my results.

I have contacted a well respected riflesmith who has agreed to work on this gun. He told me that he usually doesn't do bedding work on CZs because it is difficult to get the trigger mechanism out of the tang area and still mask off everything you need to so as to not let epoxy get into places you don't want it. He said he will take a look at it and decide if he thinks he can bed the entire action and, if not, he will simply bed the front recoil lug. Since I possess zero knowledge on the subject of bedding actions and this guy is known as a first rate riflesmith, I think I will trust his expertise to make the right decision as to what should be done.

He said he can probably get it done rather quickly, so I should have the rifle back soon and will then let you guys know if it has remedied the split grouping phenomenon. Thanks for all of the feedback to this thread and for steering me toward the idea of bedding the action.