Grendel throat question

[Dr. Lee]

Just a warning; there will be lots more questions in the future. I not new to reloading but new to the
Grendel.
I hear people talk about bullets and relationship to the lands. Just so I am clear on whats going on in the chamber, I see in the Saami drawings that the compound angle of the Grendel tapers from .2644-.2617.
So would not the bullet hit the .2617 location before touching the rifling ? Please clear up anything I've got wrong.
Cheers and Thanks.

Dr.

[LRRPF52]

On a true compound throat (most accurate throat with a wide range of bullets), the transition of the projectile into the rifling is handled more smoothly than on a conventional freebore with jump.

You get an easy bearing into the .5 degree rifling before the 1.5 degree target leade is engaged.

There are basically 2 stages to the leade.

[Dr. Lee]

So does the bullet hit the .2617" dimension before it hits the rifling ? And in reloading for the Grendel do you seat your bullets shy of the smaller part of the taper; the .2617" area ?

Dr.

[Klem]

Dr,

The bullet will touch the throat before the bore.

SAAMI specs for a compound Grendel throat is .2644" narrowing to .2617 and then angled down to a .256" bore, with .264" grooves. As you know a barrel blank is rifled from one end to the other before the reamer cuts this throat so the rifling will start to appear fairly early in the throat. getting deeper as it goes.

The first part of the compound throat is 0.1547" long, while the tighter angle is 0.1088" long.

A 120NBT is exactly .263" diameter. The bullet touches the throat in the first section and .264" groove channels will have already started, but very shallow.

I seat my bullets to max magazine length which is a lot shorter than where bullets touch the barrel. If you have a bolt gun with mags longer than your throat then it's up to you.

Ref: https://saami.org/wp-content/uploads...sting-Copy.pdf

[Dr. Lee]

Just to be on the same page, my Nosler 120gr BT's measure .264". And to put it another way so I can visualize; If you used a Hornady Lock and Load tool to get a measurement to the lands, would the bullet used with the tool hit the lands first or the tapered .2617" section first ? Is that what you mean by the bullet will touch the throat before the bore ? And another point of confusion is when people talk about such and such dimension from the lands, are they really at the lands or at the .2617" portion of the throat ?

Dr.

[Fess]

In the case of bullets with a relatively short tangent ogive (which gently curves away from the shank with no abrupt changes), the lands should contact on the ogive very close to the full-diameter shank. With an ogive that begins at an abrupt angle from the shank like a conical or secant ogive, it depends. If the angle is steep enough, the rifling would initially miss the ogive and the initial contact will be on the front of the shank. In other cases, the rifling could actually touch the ogive farther down the throat on the 1.5 degree taper section. It is important to note that I say "initial contact". That initial contact point may be tiny. If you push a bullet hard enough into the lands that you have deformed it enough to easily see the rifling impression, the area of contact may appear much longer and move a little.

BTW, this is why it is often more difficult to tune a load using a bullet with a secant ogive. Berger introduced something called a "Hybrid Ogive" in an attempt to make tuning for accuracy easier.

Chamber drawings can be a little confusing. The groove section will stay at 0.264" diameter and never get smaller. The rest of the tapered throat only affects the rifling. If you actually look at a reamed chamber, the rifling appears to emerge from the grooves, or as Klem stated: "the rifling will start to appear fairly early in the throat".

[Klem]

Dr,

As I said, your bullet will hit lands/grooves in the tapered section, at the same time. The lands/grooves are in the tapered sections.

None of this discussion is going to make any difference to group size however. If you shoot an AR you are stuck with whatever jump-to-lands you get. If you shoot a bolt gun then use the Hornady gauge to find out where it touches and back off up to 0.020".



What sort of gun do you have?

Bear also in mind that over time erosion will smooth the lands in the throat.

[LR1955]

So does the bullet hit the .2617" dimension before it hits the rifling ? And in reloading for the Grendel do you seat your bullets shy of the smaller part of the taper; the .2617" area ?

Dr.

DL:

You would seat them about .02 off the lands.

However, if you are shooting an AR-15 with a magazine, your bullet length first must fit the magazine. Most likely if they are seated deeply enough to fit the magazine, they will also be off the lands and probably a lot more than .02

If you are shooting a bolt action rifle or don't care about magazine length, most seat the bullets a little off the lands. I go with about .01 to .02. Never noticed any difference down range.

I caution you against 'hard seating' so the bullet engages the lands when it is chambered. That, to me, is begging for problems.

LR-55

[Dr. Lee]

Dr,

As I said, your bullet will hit lands/grooves in the tapered section, at the same time. The lands/grooves are in the tapered sections.

None of this discussion is going to make any difference to group size however. If you shoot an AR you are stuck with whatever jump-to-lands you get. If you shoot a bolt gun then use the Hornady gauge to find out where it touches and back off up to 0.020".

[ATTACH=CONFIG]18626[/ATTACH]

What sort of gun do you have?

Bear also in mind that over time erosion will smooth the lands in the throat.

I have a PF built 16" Krieger barreled upper.

[Dr. Lee]

I certainly don't want to be touching or jamming bullets in an AR. That's why I'm trying to get a clear picture of this chamber.

[Dr. Lee]

EC3468F0-0F39-41F8-BF4F-FFFEC5F59DD1.jpegMy NBT's measure .26385-.26395"

You might want to check your micrometer's zero.

[Dr. Lee]

Thanks for all the advice guys.

Dr.

[Klem]

I certainly don't want to be touching or jamming bullets in an AR. That's why I'm trying to get a clear picture of this chamber.

In an AR the longest OAL you will get is 58mm from a Stainless Steel mag. Mark will have reamed the chamber to SAAMI spec which is longer than the mag. You will not be able to play around with jump-to-lands like bolt gunners do. It will be a case of trying different bullets, powders and neck tensions, if you can find components.

Fattest measurement for 120NBT's this end is .2635.