Hello, I have a CZ550 in 375H&H that has produced decent accuracy with 300gr Hornady RN's. However lighter bullets have always given me poor to worse accuracy, the lighter the worse it gets. I have tried multiple powders and charge weights (RL15, Win760, IMR4350, IMR4895) with 270gr, 260gr, and 220gr bullets.

Using a comparator, I measured the actual seating depth to contact the lands with the intent of loading closer to the lands to see if the lighter bullets would shoot better. Here is a 270gr Hornady SP seated 20 thou off the lands, and no, that is not quite 1-diameter insertion into the neck:



While it fits in the magazine, that seems pretty long to me. I haven't had a chance to get to the range yet to see if this affects accuracy. But.....

- Does loading long increase or decrease pressures? (extra case volume lowers them or less freebore raises them?)

- Is this "too much" freebore for a factory rifle? Barrel cut wrong, call CZ? No big deal?

Thanks all.
Mike

Until you get to kissing the lands with the bullet, the seating depth doesn't affect pressure that much. If you seat so long that you're kissing the lands then you can see some pressure rise.

Most factory rifles are throated long, they don't want you touching the bullet to the lands because of the increased pressure it can cause. A .375 H&H is a dangerous game cartridge and as such extreme accuracy isn't the overriding factor usually, reliability is. A .375 is generally used with 300 gr bullets. I've loaded 260 gr noslers in mine for deer but if I were going to use it for what it's meant for chances are I'd be shooting 300's. Yours looks to be throated about average for a factory .375 H&H. You're most likely not going to find a factory rifle in that round where you can seat a bullet to touch the lands and still have it fit in the magazine. I'd say if your rifle shoots 300 gr bullets decently then there's no reason to complain.

The answer to your question is that seating closer will increase pressure if you use the same amount of powder.

Is it too much freebore? Actually that is not a lot of freebore when compared to the way they are chambering the RUM's and Weatherby's.

The 375h&h has to be throated for round nose bullets, so any other type of ogive is going to give very long freebores. In my remington, with the hornady 300gr btsp, I have a freebore of over a quarter inch when seated to the cannelure.

Woods. What book is that chart of seating distance and pressure from?

That chart works as you go from about 0.10 to touching I guess. But in the case of my 500 Jeffery, actually seating my Barnes 570g TSX deeper increases pressure, loading density and velocity which is actually good in my cartridge. With my load: 570g TSX, 103g of H4895 and seating depth of 3.73 (.10 from the lands) I got 2100 fps. Seating my bullet deeper to 3.63" cartridge lenght (.20 from the lands) gave me 2175 fps with the same cases, primer and powder charge. It's kind of an extreme case since the 570g TSX is so long and the Jeffery case is huge, but there it is.

Chuck

quote:

Originally posted by mrjulian_1970:
Hello, I have a CZ550 in 375H&H that has produced decent accuracy with 300gr Hornady RN's. However lighter bullets have always given me poor to worse accuracy, the lighter the worse it gets. I have tried multiple powders and charge weights (RL15, Win760, IMR4350, IMR4895) with 270gr, 260gr, and 220gr bullets.

Using a comparator, I measured the actual seating depth to contact the lands with the intent of loading closer to the lands to see if the lighter bullets would shoot better. Here is a 270gr Hornady SP seated 20 thou off the lands, and no, that is not quite 1-diameter insertion into the neck:

I have one 35cal rifle which put 9 "Light for Caliber" bullets inside a 1" square over the span of a day. The interesting thing about it was I'd Seated them to the Cannelure which created a jump of 0.264"(yes more than 1/4") and it just shot amazingly well.

quote:

While it fits in the magazine, that seems pretty long to me. I haven't had a chance to get to the range yet to see if this affects accuracy.

What you are doing might work fine in your rifle, no way to know until it is tried. Only telling you the above, because you do not always need to be near the Lands for good accuracy.

If you can grasp a Bullet with your fingers and Pull it from the Case(not working it side to side, but straight out), then the grip might be fine. You will have them protected by the magazine form lateral impacts and as long as the Recoil does not yank them out, as long as they feed well, they might do fine.

quote:

- Does loading long increase or decrease pressures? (extra case volume lowers them or less freebore raises them?)

Actually, it depends on the Shape of the Case and how much Volume is changed - if the Load is not Re-Developed. If the Load is Re-Developed and the proper Powder is used, then the amount of Powder can be different, but there should be no Pressure Problems if you are using good Pressure Indicators(CHE & PRE ). However, just changing the Seating Depth without any reguard to the Powder and Case Shape, may or may not create a Pressure issue.

The flick that Mr. Woods provided concerning the 30-06 is good for that specific rifle, that specific bullet, that specific powder, that specific primer with that specific case. Alter them a bit and the results will vary. That does not mean a valid comparison for that specific cartridge/rifle combination is useless(as is a Thingy). It is just that you have to realize the limitations.

Change the Case shape to "straight" and to a 38Spl with a 148gr Wad Cutter, then moving the Seating Depth changes the Pressure considerably. And every cartridge/rifle combination in between is different. So, it is always important to keep aware of the best possible Pressure Indicators(CHE & PRE).

quote:

- Is this "too much" freebore for a factory rifle? Barrel cut wrong, call CZ? No big deal?

If I had a rifle chambered for a 375H&H or any other really large Caliber, I would want a good bit of Free Bore. It helps hold the Pressure down when the ambient temperature goes up, as long as the person is using Factory Ammo, or "properly" reloaded ammo.

Hot Rodding Dangerous Game Rifles(DGR) works against the Reliability Factor. Plus, I do not see "me" being able to justify a long distance shot with a DGR, so it would not have to be as accurate as some of my other rifles. That does not mean your quest for excellent accuracy is wrong, I'd be doing the same thing.

Good Hunting and clean 1-shot Kills.

Guys, thanks for all the input. While I understand the accuracy goal of a DGR vs. say, a 270Win is different, it seems that this rifle is not "where it should be" based on the general consensus for a 375.

At 100yds, the 300gr bullets will group 1.5" +/- depending on what kind of day I am having. I'm ok with that.

The 270gr and 260gr group ~3", while the 220's group 6"+... this I am not ready to accept.

Seating length is the only factor I have not really messed with yet, having seated everything to the cannelure up to now while changing powders and charges.

I have loaded sets of 5rds with the a powder charge I had used previously (all of them 2-4 grains below book max), varying the seating depth to be 10, 20, 50, 100 thou off the lands.

I guess my original question was to get a general feel if I was asking for pressure trouble before starting my next experiment, and if that much free bore was out of line with a "normal" 375H&H chamber.

What I'm taking away from the discussion so far is that as for pressure trouble the answer is "maybe" and on the chamber/leade the answer is " 'bout normal".

I'm not looking to hotrod this thing, I will be happy with 270/260gr at 2700fps and a 220gr at 2900fps, each into its own 1.5" circle.

Otherwise, what's the point of having it?

Thanks again.
Mike

What a difference. Went to the range this evening and for both 270gr and 220gr bullets, when seated to 20-50 thou off the lands, I am getting 1-1.5MOA groups.

That is compared to 3-6MOA when seated at the cannelure... aka 200-250 thou off the lands.

Feeling good about the ol' CZ again.