The seating depth mystery

[tim_pope]

Hi Guys,

I'm new to reloading - I've had success and I've had some issues... It's the latter I'm looking for help on of course, but I'll set the scene so my processes make sense.

I received my rifle a few weeks back (a .308 Savage 12 FTR) and got all my reloading gear. Being new to reloading I based my initial load development on general suggestions so ended up with:

new Lapua brass

Murom primers

TR140

Lapua Scenar 155gn bullets

With assistance from an experienced reloader I took measurements to determine seating depth etc (using Hornady OAL and bullet comparator).

When using the comparator with these bullets the feeling of hitting the lands was very clear (Most likely owing to it being a new rifle I assumed)

The end result was that we were on the lands when the COAL was 2.200" So we went for a 10 thou jump at 2.190" for developing my first load.

Those more experienced may have already spotted the first little snag I ran into. The Scenars are quite long and 2.190" means we are seating pretty deep. The end result, I was only able to only put 43.5gn of TR140 max which gave me an average velocity of 2746 fps ... not going to get me out to 1000 yards (however was enough to get 1st place in my club 300 and 600 the weekend after ).

Learning of this limitation my friend kindly gave me some 155 Sierra Palmas (the shorter bullet length allowing more room for powder)

This is where the real issue begins... I went through the OAL measuring process again with nice clear feel of being on the lands like i had for the lapua (turns out this was a mistake) I came out with a COAL of 2.223" (went for a 10 thou jump again with 2.213"). Made up my test (6 sets of 5 rounds 43.5gn - 46gn TR140) and went to the range.

Fired 20 of my shots (43.5-45gn) everything looking safe pressure-wise and results initially looking pretty positive (at 44gn my ES of the 5 shots was only 11fps! at 45gn I was seeing velocities around 2787fps with my worst ES at 24fps)).

after a brief cease fire I move to the first of my 45.5gn rounds..... Bolt doesn't close

I extract the round and check it over - nothing obvious (primer correctly seated, it's new brass, visually OAL looks fine against another round, nothing else looking odd).

I try the next round - same issue (still nothing obvious) I pack up and head home wondering what I've cocked up.. maybe I messed up the seating depth somehow...

So bullet comparator out, recheck all the rounds I have left... 2.213" as they should be (I have a micrometer seating die and spend the time to measure after each seating, adjusting until perfect).

So why did my first 20 shots go in fine, but the next 2 suddenly fail - my thought is that perhaps I am right on the lands.. those first 20 were fine, but as the barrel became warmer and expanded along with having more fouling that effectively the lands became tighter, stopping the next rounds from fitting.

Does anyone have any thoughts on this... I found it odd I was fine for 20 shots then suddenly not able to close the bolt - so would love to get to the bottom of this.

Going on this theory I decided to remeasure the OAL and this is where I noticed I must have miscalculated the first time round - it seems with the Sierras the part of the ogive that touches the lands is much steeper than what I had with the Scenars - which means contact is not as clear cut.

Having a general play with the gauge and some spare bullets I could see where the lands were leaving scratches, so I covered a bullet in felt tip pen and worked on measuring the OAL where the pen was no longer being scratched off (I figured this should give me a much clearer picture of when we are actually touching the lands vs feel (which clearly was difficult to do reliably in this instance)

My new technique resulted in a COAL measurement of 2.191" Which is 0.032" difference from my original measurement! Which has me thinking perhaps my new method is being far too cautious now - it's also shorter than what I had for the Scenars which seems a little odd.

I've reseated the remaining rounds to 10 thou off this new depth and checked things through on quickload... nowhere near max pressure (in reality I don't think I'm going to get the velocity I need for 1000 yards with this powder fullstop, but I'll test anyway) and certainly don't sound like compressed loads) ready to test at the weekend.

To try and compare my seating to my friends I figured I could use the difference between OAL of the Scenars and SMK in his vs mine as a direct comparison.

In his, the difference is that SMK's OAL is 0.012" longer thank the lapua

In mine I had initially as 0.023" longer (so clearly too far) however now I am actually 0.009 less than the lapua... suggesting I'm completely off-base again (although from a safety perspective a long jump isn't going to do harm) - yet as mentioned above, there is clear scratching on the bullet of seated any longer - so in truth I'm not sure what to make of it.

So here's the questions neatened up after my prattling on

Does my suggestion of why the bolt stopped closing hold any weight - if not I'd really like to know what happened (just one of those things really bugging me)

Do you think I am being too cautious with my new OAL measurement?

Do any of you have your own data for COAL when loaded with Scenar - Coal when loaded with Sierra Palma... I'd love to get a few more to compare and see what the general difference is as this should allow me to derive the seating depth (even though you have a different gun the ratio of length difference should be fairly universal... I think)

Thanks for any and all help

Tim

[Laurie]

The Stoney-Point / Hornady system involving a modified screw-on case is often seriously inaccurate when used with a minimum SAAMI spec chamber. I've long used the system for factory sporting and historic military rifles, but found that as soon as I moved onto custom rifles and / or rebarrelled rifles with minimum SAAMI spec chambers, the results are wrong, always an over-high CBTO reading. Depending on hows 'tightly' Savage cuts its FTR rifle chambers, this may well be your problem.

 

The alternative systems involve use of a cleaning rod and collars inserted from the muzzle - The Gun Pimp will explain that method as he uses it for everything - or the Sinclair International system which uses a short rod and collars inserted from the rear, this needing an unsized case previously fired in the rifle with spent primer still in situ. See: http://www.sinclairintl.com/userdocs/learn/Sinclair_Seating_Depth_Gage_Inst.pdf

 

A third method, one that I use, is to take a new or resized unprimed / deprimed case, check it chambers without any resistance on bolt closure, seat a bullet in very long and attempt to chamber the inert round. The first attempt should see you unable to move the bolt far enough forward to be able to turn the handle down. Keep seating the bullet deeper until you can close the bolt but just feel some resistance on final closure. Pull the bullet, reset the die 20 thou' shorter, and seat a new unmarked bullet and try again in the rifle - extract and look very carefully for marks made by the lands around the ogive - use a felt tip marker, better still a good quality and large magnifying glass. If you're just kissing the lands, they leave a series of very narrow lines that can be difficult to spot. Go fractionally deeper, using 'clean' bullets as required until you just get a combination of no resistance and no marks, then reset the die by your ten or 15 thou' deeper to get the desired jump. (Alternatively, regard a barely visible line as being 5 thou' 'in' and reset at your desired jump + 0.005".)

 

For best results to maximise the 'feel', remove the striker / mainspring assembly from the bolt body beforehand. Unfortunately, the Savage PTA still has some camming resistance on bolt closure / primary extraction, variable depending on how good the bolt timing is from the factory - my trio are noticeably different. Using a Barnard P action less internals sees no resistance to bolt turning at all making it very easy to feel even marginal chambering resistance caused by mild bullet to lands engagement.

 

Keep your inert round at that setting as a 'seating dummy' and use it set up your die and in conjunction with a comparator for fine-setting. backing the die setting off the seating dummy and screwing the stem down onto the bullet usually produces rounds that are 0.003-0.005" longer than the CBTO of the dummy so you need to check the results on the first rounds you load and fine-tune the die setting - micrometer top dies are a godsend in all of these processes.

[Chris-NZ]

The Stoney-Point / Hornady system involving a modified screw-on case is often seriously inaccurate when used with a minimum SAAMI spec chamber. I've long used the system for factory sporting and historic military rifles, but found that as soon as I moved onto custom rifles and / or rebarrelled rifles with minimum SAAMI spec chambers, the results are wrong, always an over-high CBTO reading....

 

 

I'm having trouble getting my head round that Laurie.

 

I -assumed- the proprietary "modified cases" come -full- length resized, ie well clear of generous factory chambers. If this is the case, the resultant measure should if anything produce an under-size measurement for a generous chamber and a more correct measure for a tighter chamber. That's why I make modified cases from brass fired in the same rifle.

 

Where does my logic fall down?

[JSC]

.

[JSC]

 

2.277 (0.010" into rifling) SMK 2156

2.357 (0.010" into rifling) Lapua Scenar 155gn

 

These are measurements I took when working up loads for a Remington 700. They are COALs (from ogive). Might be of some help?

 

Remember that your particular datum point for when the bullet touches the lands is just that. It's just your datum point you measure everything else from, so as long as you figure out a consistent method to do that (and remeasure it from time to time as the throat erodes) then you should get consistent results.

 

One way I did it with a Stoney Point gauge was to allow the bullet to slide into the lands by slowly bringing the rifle to a vertical position, so there was no extra force pushing it into the rifling other than the weight of the bullet itself.

 

One other thing I'd say is if you couldn't close the bolt and you're sure it was the seating depth at fault, I'd be wondering if I had too much neck tension on those rounds...

[Laurie]

 

 

I'm having trouble getting my head round that Laurie.

 

I -assumed- the proprietary "modified cases" come -full- length resized, ie well clear of generous factory chambers. If this is the case, the resultant measure should if anything produce an under-size measurement for a generous chamber and a more correct measure for a tighter chamber. That's why I make modified cases from brass fired in the same rifle.

 

Where does my logic fall down?

 

 

The factory modified cases apparently don't chamber 100% in a minimum spec chamber. That is, the case-head face will be further from the start of the lands than in an actual loaded round, so one gets an over-long reading. Using a fireformed case should put that right, but I decided a long time ago that this method is not 100% foolproof even though one may not see where the problems arise. That is a point which Sinclair International makes both in its sales blurb for its rod base system (you could say of course that they would say that!) and in its blog bulletins.

 

Even at its best, the system rarely gives 100% accurate readings, but gives them close enough to work OK providing people don't get into a mindset that says they can set COALs that are say 0.003" 'out' and that every bullet ogive will be that tiny distance off the lands exactly.

[jcampbellsmith]

I neck turned some once fired 6XC cases recently and gave them to Grant Taylor so he could drill and tap them for the Hornady OAL tool. Regards JCS

[Brillo]

Here's a method adapted from Laurie's as suggested by several other shooters and it works.

• Take a fire-formed case that you know seats easily.
• Cut three slots in the neck with a fine Dremel or very fine hacksaw equal distances apart. Clean with wire wool to remove any burrs.
• Resize the case but only the top 2-3mm of the neck.
• Seat a bullet and then ease it slightly so the bullet will push down easily with slight pressure.
• Now colour the bullet with a felt tip pen or smoke it over a candle and chamber the round slowly.
• Remove the round ensuring it doesn't eject and look for rifling marks. If the neck tension is correct there should just be a slight mark where the bullet touched the lands.

A further adaptation of this involved grinding off the rim off the case web to ensure the bolt will not eject the round.

Once the round is chambered, remove the bolt and gently tap the round out with a cleaning rod.

It's a bit like brain surgery but with this you get to practice until perfect.

[fizzbangwhallop]

I use a method similar to Laurie/Brillo....

 

• Clean the inside of the neck of a fire-formed case with a bristle/phosphor b. bore brush;
• Deform the case mouth slightly to give just enough grip to hold a bullet;
• Just insert a bullet enough to sit straight and felt tip marker it ensuring it's covered where it meets the neck of the case;
• Carefully insert this into the chamber and gently close the bolt on it;
• Open the bolt and remove the case without allowing it to eject;
• If the bullet remains in the lands just tap it out with a cleaning rod from the muzzle;
• The felt tip ink will be scraped by the neck and will show exactly how far the bullet was pushed in before meeting the resistance of the lands should it be pulled out either fully or partially on extraction;
• Dead easy to repeat x times to confirm the measurement.

 

Spend the money saved from buying an OAL gauge on a decent comparator such as one from the ShootingShed.

 

Cheers

 

Fizz

[tim_pope]

Hi Guys,

 

Thanks all for the input - some great little tips I'll be adopting next time to try and improve things

 

JSC thanks for the comparison COALs - this adds more support for my new measurement rather than the old (Your OAL for SMK is less than Lapua) I shall also give your tip of using the bullet weight to gently ease it into the lands a go - sounds like it'll be a bit more gentle than my fat fingers.

regarding your comment on neck tension - this is brand new Lapua brass and I have not resized them, I also saw visible scratching on the bullet from the lands - So I assumed this was likely ok tension? (which is also why i'm still perplexed why the first 20 were ok)

 

In regards to methods I think a lot of it really comes down to the person measuring the 'feel' - My friend and I actually used both the gauge and the rod/collar method for the original lapua measurement - From memory I believe we saw less that 0.005 difference between the 2 techniques (so from a safety perspective you are going to be a min of 5 thou off lands even if you take the longer OAL) obviously if you are playing with jump - then you'd likely want to refine the method a bit more (as I think I will be doing after this).

 

If anyone has any further tips, shed light on perhaps why (if the rounds were made up the same) the bolt stopped closing on the 21st shot, or can provide more SMK vs Lapua OAL's it would be much appreciated.

 

Thanks

 

Tim