Interesting numbers loading 7mm mag

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Interesting numbers loading 7mm mag

Post by horseman »

I know pretty much all of us (?) hate math but I ran into an interesting issue while loading for my "new to me" 7 mag.

Components:

RP once fired brass (not from my rifle) full length sized with Forster BR sizing die. Did not "bump" shoulder as the brass cycled fine in my chamber. Brass was trimmed to minimum of 2.490 per Speer reloading book. All were within 2 thousandths.

Bullets are Speer 145gr. SPBT (hence the Speer manual) load data is unimportant as this is about OAL, distance to lands etc; (however the load data could be affected by differences in the seating depth)

Manual says OAL as tested is 3.280 using this bullet.

So I proceed to load up 12 rounds using this number (I know base to tip measurements can vary a bit because of the lead tip irregularities but should still be within a couple thousandths) using Forster BR dies and a co-ax press. (means nothing in the overall scheme of things) just like most would do (I believe).
Well guess what? Rounds wouldn't chamber in my rifle.... :shock:

So out comes the Hornady "head space" gauge (really a head space comparator) set up to find distance to the lands from the ogive of this bullet. Measurement comes out at 2.730 (this number means nothing by itself except as it applies to the other measurements taken with the bullet and case comparator) The "head space gauge" uses a modified case made by Hornady for measuring so you need to compare it to the cases you're using (shoulder location) The modified case is 2 thousandths shorter in this area than the cases I'm using so that has to be taken into account when figuring seating depth. I decided to seat 20 thousandths off the lands (just a place to start) so subtract 20 thou from the 2.730 and we have 2.710 PLUS the two thousandth difference between the modified case and the once fired case, so we're now at 2.708. This should put us 20 thou of the lands OF MY RIFLE.......told ya there would be math involved.... :lol:

Bullets were seated using the above mentioned Forster BR seating die and all came out within a thousandth of each other measured with the Hornady bullet comparator. Here's where it got interesting to me (really all of this was/is "interesting") when I measured the OAL of my finished rounds with a good set of calipers (aerospace) they come out at 3.198... .082 thousandths SHORTER than the book shows as tested. Even if you load to the lands on this rifle you'd still be somewhere around 60 thousandths shorter than their number. Wonder what kind of chamber they were using as a test platform? I did measure some factory Federal 150gr SP to compare and these were 20 thousandths SHORTER than my reloads. I've come to the conclusion that some method of making these measurements is more important than what I USED to think. I know I'll not reload another bottleneck rifle round without first checking this. Does anyone see anything I missed or did incorrectly? +corn
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Re: Interesting numbers loading 7mm mag

Post by akuser47 »

I cant add anything to help, but in to learn what I can. +corn
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Re: Interesting numbers loading 7mm mag

Post by Ranch Dog »

Interesting. Have you tried chambering a dummy cartridge at 3.280"?
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Re: Interesting numbers loading 7mm mag

Post by horseman »

Ranch Dog wrote:Interesting. Have you tried chambering a dummy cartridge at 3.280"?




Yes I did. When I reloaded the first 12 rounds I seated the bullets to the book "as tested" OAL, which was the 3.280 number. Wouldn't chamber. Pulled the bullets from those rounds, rechecked trim length on the cases, pulled decapping pin from size die and resized the cases to reset neck tension. That's when I started checking things with the Hornady head space (?) gauge and bullet comparator. Truthfully when I bought that particular equipment I really didn't figure I'd really need to go that far with my reloads thinking it was more for "target" shooters but I've changed my mind after this session. The whole question of OAL per book and what I found for could be something as simple as a (typo) in the Speer manual. By the way the rifle in question here is a Savage model 111 so I'm pretty sure it isn't a "short chamber" involved. Especially after measuring the factory Hornady bullets and finding they are actually (if my figuring is correct) 40 thousandths off the lands of this rifle. That would seem reasonable from Hornady as to make sure that they would work with any 7mag. Seems logical to me anyway.


edit to include: Just checked a Sierra manual and the OAL for the Sierra 140 and 150 grain bullets show as 3.240 vs my 3.198 (let's just make that a 3.200) of course the Sierra would have a different ogive so those numbers really don't say much. I do have some Sierra 160 HPBT so I could load up a dummy using their numbers and see how they chamber. Get back to ya on that later today if interested.
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Re: Interesting numbers loading 7mm mag

Post by Ranch Dog »

Are the bullets at the specified diameter?
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Re: Interesting numbers loading 7mm mag

Post by Ranch Dog »

The tools are great but the only way to know if your chamber has all it's intended features is through a chamber cast. Something isn't right.
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Re: Interesting numbers loading 7mm mag

Post by Ranch Dog »

Ranch Dog wrote:Are the bullets at the specified diameter?
Besides the .284" diameter, a Speer #1628 should be 1.165" long. That is from the QuickLoad database.
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Re: Interesting numbers loading 7mm mag

Post by horseman »

Just loaded a Sierra 160gr. HPBT dummy round. Set the OAL (base to tip, using calipers, like most would do) at book setting of 3.230......chambered just fine. I WILL check this bullet with the gauges I have and see what the MAX length to the lands of my rifle is. Thanks for those numbers RD, I'll check that.



checked several of the Speer 145gr SPBT bullets with calipers.....all were .284 and length was 1.152-53. Maybe ignorance IS bliss....as long as it fits.... :)
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Re: Interesting numbers loading 7mm mag

Post by Ranch Dog »

Horseman wrote:Just loaded a Sierra 160gr. HPBT dummy round. Set the OAL (base to tip, using calipers, like most would do) at book setting of 3.230......chambered just fine. I WILL check this bullet with the gauges I have and see what the MAX length to the lands of my rifle is. Thanks for those numbers RD, I'll check that.
It almost seems that the bullets are "fat" or the ogive has been bumped during manufacture. Without a drawing, there is no way to measure if the start of the ogive or its radius is wrong. I'm pointing my finger at the bullet. Do you have another box of the #1628s? I don't think Speer uses lot numbers on their boxes.
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Re: Interesting numbers loading 7mm mag

Post by horseman »

Ranch Dog wrote:
Horseman wrote:Just loaded a Sierra 160gr. HPBT dummy round. Set the OAL (base to tip, using calipers, like most would do) at book setting of 3.230......chambered just fine. I WILL check this bullet with the gauges I have and see what the MAX length to the lands of my rifle is. Thanks for those numbers RD, I'll check that.
It almost seems that the bullets are "fat" or the ogive has been bumped during manufacture. Without a drawing, there is no way to measure if the start of the ogive or its radius is wrong. I'm pointing my finger at the bullet. Do you have another box of the #1628s? I don't think Speer uses lot numbers on their boxes.

No, just the one box of 1628's. You know you just reminded me of something....I originally bought these to load in a 7x57 Mauser I had and I remember that after seating these bullets to (whatever the number was for the 7x57) I had a really hard bolt closure so I re-seated them a bit deeper (don't recall off hand how much) until no bolt pressure. Was using a starting load so the pressure didn't worry me much. I think you've hit on the problem with this batch (?) of bullets being the culprit. I have no way to check this but it does make perfect sense.
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