Seating depth for a newbie

[timberelk]

Was just about to load up some progressively hotter loads to go try to shoot at the range tomorrow. But I got to looking at the nosler book and they said with the BT bullet and varget, the most accurate load was with 38 grains. So it should be fairly accurate in my gun I would think

Well last weekend I shot 6 or 7 of 38 grains and 6 38.5 grains and they produced the below groups (ignore the words new and old and the large 50 cal bullet holes). So approx 2" group and I got to thinking instead of messing with the powder what if I keep the powder around 38.5 or 39 and started to mess with the seating depth because it will result in potentially dramatic grouping difference?

My lands are approx .25" from the nosler OAL. So I can't get close to the lands but I can start with .020" farther out, then maybe -.020" (closer in) and then maybe .040" farther out and then maybe -.040" (closer in).

What's your guys thoughts?

 

[Tidus56]

Why cant you get closer to the lands? Are you worried about exceeding the OAL posted in the book? Or do you have a DBM that doesnt allow more length? How did you measure for your lands? Finally what bullet are you shooting?

 

[timberelk]

Why cant you get closer to the lands? Are you worried about exceeding the OAL posted in the book? Or do you have a DBM that doesnt allow more length? How did you measure for your lands? Finally what bullet are you shooting?

Nosler Ballistic Tip 140 grain 7mm.
I found the lands by taking once fired brass from my chamber and barely placing a bullet inside the case mouth and then slowly chambering it so the rifling would push the bullet into the case mouth. I did this about 20 times and came up with my lands at 3.002" OAL.

I use OAL instead of my Hornady ogive guage because the nosler BTs only fluctuate by .001-.002" when you measure base to tip.

Can't get to the lands because loading the bullet out .203" ish inches would be well beyond my internal magazine and I'm worried about how much the case mouth is holding onto the bullet.

So when the bullet seated for an OAL of 2.800 the bullet stick outs about .890" and the total bullet length is 1.365" so that means right now the bullet is seated .475" inside the case mouth.

I've read that I want at least .284" because this is a 7mm caliper (.284 caliber)

So I guess I can really only Seat the bullet out .191" further out or I can go deeper if that would help? Idk?

 

[timberelk]

1. I apologize but I did another lands test... took twice fired brass (was FL sized between the first and second firing) and I squeezed the case mouth with my fingers so the bullet fit really tight into the mouth.
   
   Took the 7 cases and tried it once with each case and I came up with my lands at 2.908-.2.910. So that means with loading to the manual I am currently .110 off my lands.
   
   So to be .040" off my lands I would have to seat the bullet out .070"out further. That means the bullet would be seated .312" approxinside the case mouth and I'd be.040 off the lands
   
   
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[timberelk]

Measured my mag and it's about 2.895. So if I load the bullets .040" off the lands my OACL would be 2.870 so I would have about .025" of room in the mag

 

[FatOldMIHunter]

How much bearing surface of the bullet is inside the case mouth? The boattail doesn't count as bearing surface. I've heard several rules-of-thumb about minimum seating depth. The most conservative is that you need one bullet diameter of bearing surface. The least conservative is that you need at least .080". Of course neck tension plays a part. I use a minimum of .150" with 140 gr boattail bullets in 6.5mm and .200 with 162 gr bullets in 7mm.

 

[timberelk]

How much bearing surface of the bullet is inside the case mouth? The boattail doesn't count as bearing surface. I've heard several rules-of-thumb about minimum seating depth. The most conservative is that you need one bullet diameter of bearing surface. The least conservative is that you need at least .080". Of course neck tension plays a part. I use a minimum of .150" with 140 gr boattail bullets in 6.5mm and .200 with 162 gr bullets in 7mm.

My bullet is sticking out of the case mouth about .875 (hard to measure exactly) and the total length of the bullet is 1.265 so I believe the bullet is inside the case mouth by .390 but that includes the boat tail

That is with loading the bullet .025 of the lands so my OACL is 2.885

 

[FatOldMIHunter]

You run the risk that the bullet will be not gripped tightly enough and not prevented from tilting in the case mouth if you don't have enough bearing surface. This is very important when hunting due to the rougher handling that rifles can receive. Also, recoil can cause seating depths to change for cartridges in the magazine when there isn't enough bearing surface or neck tension.

 

[DUSTY NOGGIN]

Measured my mag and it's about 2.895. So if I load the bullets .040" off the lands my OACL would be 2.870 so I would have about .025" of room in the mag

.025 magazine well ,would be just right , gives a little clearance

so the closest you get to your lands is .040 , no problem

i personally work on powder
first do optimal charge weight need to find the plateau plateaus of the one shot ladder and making sure you find pressure limits during this test , by loading to maximum or better

!!! knowing you will be pulling disassembling everything charged higher than your pressure signs as soon as you get back to the press !!!

then would suggest a full array of powder from minimum to maximum shooting at each set at its own target at 200 yard with that 7 ( or 100 yard), then once you find the hourglass effect and can see the obvious trend keep notes , then you will have a visual measurement of your nodes ,so to say

this is just me and my chaotic mind thinking :
seating does 2 things and changes pressure going both directions -- the farther away from the middle ground that faster those changes happen

*seating deeper into the case , reduces pressure by giving bullets more momentum as it swages itself into the rifling/lands
*seating deeper into the case , ALSO increases pressure by creating less volume in the case capacity ( but changes pressure faster )
* seating longer ( assuming you do not touch lands ) increases pressure ( more so with all copper bullets ) - has to start swaging as soon as it starts moving
* seating longer ALSO reduces pressure by increasing case capacity ( skewing the graph of pressure - so changes happen faster over same amount of time )

pictures this , a skateboard with a small pebble right in front of the front wheel - if you get a running start and try to land on that skateboard and roll over it you may end up landing on your face , but if you give that skateboard a rolling start of 6" or so inches and do same thing you will roll right over that pebble ... you will most gun manufactures make the magazine shorter than you could ever touch the riflings ,

almost like getting free throat erosion with every purchase !!!

so i think in your situation with what you have described above you will get the best of both worlds by seating as long as your magazine well will allow ( decreasing the pressure per given area of case capacity ) and adjusting your overall pressure by charge weight

IMO staying away from both factors will lessen pressure changes and the closer you get to either factor increases the variation of those changes

i believe the seating depth test is really just seeing how far away you are moving the skateboard away from the pebble , knowing it will roll over it reliably

knowing that your ballistic tip bullet is a very soft bullet compared to others , it will swage a lot easier than other bullets

... i think that the .040 jump you are at is very good and all other pressure adjustments can be made with careful precise power charges ( to the 10th of a grain )

 

[timberelk]

Got to shoot the OAL 2.885 this morning. Changing the bullet jump from .113 ish to .025" made a huge difference and shrunk my groups in half on some of the groups.

Looks like 39 grains of varget shot best but I'm going to continue increaseig the powder in hopes I find another accuracy node near max pressure/powder but I think I will stick with 2.885 OAL because it clear my mag by about .010-.015 and gives me approx approx .390 if neck/bullet contact (minus the boat tail so I would guess I'm about .300 neck tension)

 

[jrock]

I would seat out as long as you could and leave 0.02" of space between the mag and the bullet tip. Also, I believe there are ways to get a more defined distance to land. A search on this site should yield some good methods that you only have to do once. Not 3 or 7 times. I like the Nosler book because it shows the most accurate value but take it with a grain of salt. Every barrel is different. I have bullets seated 0.10" off the lands that shoot 1/2". Depends on the bullet. Starting 0.02" from the lands is where I start if I can reach the lands. If not, then I start with the book COAL and work form there with powder charges.

 

[timberelk]

I would seat out as long as you could and leave 0.02" of space between the mag and the bullet tip. Also, I believe there are ways to get a more defined distance to land. A search on this site should yield some good methods that you only have to do once. Not 3 or 7 times. I like the Nosler book because it shows the most accurate value but take it with a grain of salt. Every barrel is different. I have bullets seated 0.10" off the lands that shoot 1/2". Depends on the bullet. Starting 0.02" from the lands is where I start if I can reach the lands. If not, then I start with the book COAL and work form there with powder charges.

I removed the firing pin and attempted to remove the ejector pin but when hitting the pin that holds the ejector in with a punch, I was actually starting to bend my punch so I stopped and left the firing pin out and took 6 or 7 twice fires cases, pinched the case mouth with my fingers until the bullet fit very tight into the mouth of the case then left it extremely long and chambered the round letting the rifling push the bullet into the case. I did this about 15 times. A few times I got around 3.002 because I believe the rifling pushed the bullet in and then held on to it for a brief second while I was pulling the bolt out which would result in a longer OAL measurement but 90% of the time I was between 2.909-2.912 so I seated to 2.885 and got decent results