How Much is Too Much for group accuracy

[rhouser]

I cast 375449 lymans with a brand new mold over 2 days (testing and playing) and at varying tempratures (testing for what the mold likes). I took a few (about 100) and loaded them up with varying powders and loads and primers and shot them at 100. (At 38-55 velocities from a 10 lb 375 H&H). The results were what would be expected in testing, they varied.

Now to my question and my point: Of the loads that did well, there was a random double or triple group occuring --- 3 in one tight clover leaf and the next 2 or 3 in another tight clover but 2 inches higher. I did check my scope mounts and my action screws but didn't do a whole rifle shakedown because I have another theory.

I weighed out my finished lubed bullets last night and they fall into 4 distinct weights (kinda like the different temperatures and days and an alloy shift while testing the mold). There is a 4 grain difference in weight between the top and bottom group of bullets and about a 1.5 grain difference between each of the cast groups.

Is the weight difference of 4 grains between two sets of bullets enough to create a 1.5 inch difference between two groups? I have sorted them and will reshoot my best loads with bullets of the same weight to see, I was just looking to hear from experienced shooters because I have to wait a week to shoot and this is making me crazy.

Opinions are welcome and thanks in advance.

rc

[felix]

Yes!!!! ... felix

[Blammer]

Yep, the wt matters and that's why you got what you got. I get the same thing when I'm load developing.

Here's the fun part. Your best groups may be with a different wt than you are expecting. :)

[44man]

Yes!!!! ... felix
I agree. Large, heavy boolits are not affected so much.
Yet one other thing to consider is your recoil management. Any change in hold will also string shots. Even a few ounces of your cheek to the stock will do it.
Shooting every single shot the same has to be the hardest thing on earth to do. We are at the mercy of our bodies. I am here but I think my body never left the bed! :confused:

[Larry Gibson]

If you are using a double cavity mould is the weight variation caused by the 2 different cavities? Sometimes if the sprue plate will lift up when the 1st cavity is filled and then the shank of the 2nd cavity bullet will be slightly longer and the bullet thus heavier. You should be able to see if this is happening by holding it up and looking between the sprue plate and top of the mould before cutting the sprue off.

As mentioned, that much weight variation can definately be the cause of the 2 group stringing.

Larry Gibson

[leadman]

You said you used different powders and charge weights. This normally will cause a shift in poi. I have seen as much as 6" at 100 yards. Most of the time the poi will change in 2 directions.
The variance in the boolit weight can also cause a change in poi.
Find a couple of good groups from your shooting and load some more of the same with weight sorted boolits and shoot them again. This way you can eleminate any load that might have been a fluke.

[montana_charlie]

When developing loads, all parameters must be as perfect as you have capability to control them, and bullet weight is the most controllable.

At least be certain that every bullet that will go into a given group is the same weight as it's group mates.

[sqlbullet]

When I cast rifle boolits I am looking for ±.25%. In you mold that would be a 1.3 grain swing from top to bottom.

What I do is sort my boolits by weight using a digital scales. The scales is accurate to 1/10 grain. I have learned over time that my mold will have a sweet spot around 195.5 grains, so generally I end up keeping anything between 195 and 196. This yields a loss rate usually of about 15% if I have done well at the pot.

The rejects go back.

For handguns I don't care so much.

[runfiverun]

airc the 375449 is about 270 grs.
4 grs is under 2% weight variation.
you can however be shooting the 2 cavities to different poi.
you could make a dimple in one of the cavity's and see.
i had a little issue wth mine making slightly rounded driving bands on one side of one cavity that would cause flyers.

[rhouser]

Larry and run5run: I was trying out the new mold. Day 1 was running my basic alloy (about a lyman 2 (minus). I was casting at about 690 to 700 and had mostly bad bullets. I kept about 25 that "were shootable". I recleaned the mold, uppd the temp to 725 and things got better. I kept about 40 bullets that weren't too bad.

Next morning I added a biscut of linotype and 2 biscuts of elevator weight lead to my basic metal to add some tin and some antimony and added back enough of my base metal to bring it to a full 20 lb pot. I upped the temprature to 760 and the mold filled well. I kept about 100. These all went into the same bag with all the "keepers". I applied 45/45/5, sized them to .377, gc'd them and reapplied 45/45/5. I just poured out about 100 from the total and loaded them indiscriminatly. Because I used different alloys and different temperatures, I am not alarmed by the 4.5 grain difference top to bottom. I will check the 1 - 2 position weights on the next cast.

Leadman: I was shooting 10 rounds per load test. The ones that grouped well (meaning MOA or less) were really 2 inch groups with 2 of 3 cloverleafed groups in it. I believe I may have a good load if I keep the bullet weight fenced down to a single weight group.

sqlbullet: Thanks for the info on a "standard" (±.25%). This will give me something to "shoot" for.

All: I am just finishing my first year of casting so I have much to learn. Thank you for your help and all you do for this forum.

rc

[williamwaco]

Recoil management was mentioned but I didn't see any mention of holding the weapon or resting it on the bags.

It is not at all unusual for poi to change by that much just by changing the way you hold the rifle or by how you place the forend on the bags. This is the reason for the forend stop found on many mechanical front rests - to allow you to position the forend on the rest exactly ( pretty much ) every time.



This happens to me quite frequently when a group is interrupted by a cease fire. I will wind up with two distinct groups, one for shots fired before the cease fire and a different group an inch or two , or three - away for the shots that were fired after the cease fire.

I find this to be so common that when a group is interrupted by a cease fire, I do not finish it after the cease fire. I just move on to the next target.

[Gohon]

Is the weight difference of 4 grains between two sets of bullets enough to create a 1.5 inch difference between two groups?

Not the way you're describing it. Always a tight 2-3 shot group and then up two inches for another tight 2-3 shot group. What would be the odds of the first group always weighing the same and the next group of the same weight but different than the first weight? Pretty astronomical wouldn't you think. Ever see a group drop down two inches from the first group.....probable not. If the 4 grains really made that much difference then your grouping should be spread out. You've got something else going on other than difference in bullet weight.

[2Tite]

A 4 gr. difference at only 100 yds giving a 2" difference. MMmmm....I'm with 44man. I'd look seriously at benchrest technique first and try one cavity at a time. Mark one of the cavities with a punch mark so you can sort the bullets after you cast. Just my 2 cents worth and worth everything you paid for it.

[rhouser]

2Tite, the groups were about an inch apart. Could be a rifle issue also (double grouping). I shoot .30's with my 6br and can shoot MOA at a grand with my 6.5 and 300 Rum on a good day. I am using a fairly good rest and rear bag (not my Hart) but same rig supports an easy MOA with my 38-55's and 45-70's at 100 with RD's and LaseCast commercial.

All: I agree that the double grouping could be many things. I flat had some lost bullets at 100 as well.

The gun is new to me, and, I am a one year casting veteran. I still have lots of questions.

A couple that never came up with me before are: How Big a Grain Deviation should be considered acceptable and what happens if I build the same load with two bullets 4 grains apart.

The answer seems intuitive to you as veterans but with cast bullets in a 375 H&H it is not intuitive to me.

When I was much younger and shooting bench, I used to lathe turn my bench brass for my tight neck bench gun, use wilson arbor dies and change loads mid match to keep up with the temperature. I would NEVER use bullets that were not matched.

Now I stuff oversized lead Boolits into cases stretched to accept them and am having a Ball. I find more pleasure in hitting a dinger at 200 with a 100 year old rifle than in worrying about a perfect string at 600 with a high tech rifle. Guess we all get old.

Again, thanks to all. My questions are answered. rc

[williamwaco]

A 4 gr. difference at only 100 yds giving a 2" difference. MMmmm....I'm with 44man. I'd look seriously at benchrest technique first and try one cavity at a time. Mark one of the cavities with a punch mark so you can sort the bullets after you cast. Just my 2 cents worth and worth everything you paid for it.

I sort them as I cast them.

Place a high-tech bullet sorting separation device ( stick ), along the middle of your drop pad. Drop the bullets on the front of the pad then push the front one to the left and the back one to the right. This slight delay also helps keep the mold from overheating.

When you run out of space on the pad, transfer them to a holding box with your spoon. You will soon wind up with two boxes of bullets, each box containing bullets from the same cavity.



.

[Larry Gibson]

The large weight swing is probably induced by the 25 bullets in the bunch from the softer alloy before the lino and tin were added.

Larry Gibson

[303Guy]

I have a single cavity solid mold and when I got to sorting the boolit weights I found there was a quantum difference between each variance. Two causes for this, my electronic scale has a resolution of .2grs and second, the variance was being caused by something under the sprue. When that changed, so did the boolit weights.

[runfiverun]

lot's of good answers here.

i have a 22-250 that was throwing two groups,i changed the scope...nope still 2.
went through the bedding....nope.
checked the bbl gap...nope.
tried a bit of pressure under the bbl...nope.
at the tip...nope.
looked the trigger over and checked the pull weight...and still,, nope.

finally figured out it was the height of the scope to the stock versus the height of my cheek to my eye.
i was slightly moving my head occasionally just enough....

[rhouser]

No rain for Saturday, so I will take another run at this with the bullets sorted by weight and like weights fired together as a single group.

As runfiverun and others have pointed out, it may the the gun or the shooter.

If I still can't get a single group, I will then TI the rifle, and may bring in another shooter to try the load. I will also load some "eggh" jacketed hornady 225's in a load proven to shoot in this rifle. If I can shoot these but can't shoot the cast load, then, I will dump this boolit/powder combination and start again with another powder.

thanks to all. rc