6.5CM bullets - large jump to the lands. Is this normal??

[chaz]

Hi folks,

I'm after some advice regarding the seating depth of my chosen bullet heads in my Lapua 6.5CM brass.

I was hoping to seat the bullets around 15 to 20 thou off the lands as a starting point.

However, if i do this, only around half of the case neck will be holding the bullets in place, due to the boat tail.

I was told some time ago to have at least the width of the caliber being held by the neck. If i do this, my Lapua 139 grain OTM Scenars will be seated 94 thou from the lands!

Then my Sierra 130 grain TMK's will be seated 69 thou from the lands!

Is this normal?

I'll be using the rounds for both target shooting and hunting (TMK) and appreciate that while target shooting the rounds will be kept "safe" and they won't get knocked about. In fairness, i'm also careful with them when out shooting quarry. With this in mind, should i still stick to the rule about seating the bullet heads to at least the bullets width?

Some constructive advice would be appreciated.

Cheers.

Chaz.

 

Mods - I've just realised i may have put this in the wrong section, feel free to move it.

Thanks.

[VarmLR]

This doesn't sound right to me Chaz. I have the same rifle and use the same bullets and my COAL to lands with the 139 Scenar is 2.872", and with a 30 thou jump, I have an ideal seating with the boat tail starting at the very base of the neck, ie with the bullet well seated so that all of the neck is supporting the bullet. Is it possible that T3's vary in throat dimensions by that much??? I have checked and double checked my seating. Similar story with the Hornady 140 SSTs seated at the same jump.

[snakeman]

No idea on the dimensions of the 130g TMK for 6.5mm BUT, as an indication...the 155g TMK for .308 I am seating ogive 125 thou off the lands....(2.875 COAL)

 

That's the seating depth from where the bullet performs best and profile of the TMK class is different to anything I've used before.

 

N.B.

Incidentally, I was advised to seat the 155g TMK at a COAL of 2.800 by none other than Sierra Bullets (Dwayne)...When seating at the Sierra recommendation of 2.800 COAL the ogive sank too deep. I emailed Sierra and advised of this and they promptly apologised following their own test of the same bullet. Subsequently they advised a COAL of 2.840

[chaz]

Thanks for the comments both, I was being dim, and measuring from the wrong point.....

Now sorted, with the help of VarmLR! Much appreciated Paul!

Cheers

Chaz

[VarmLR]

Thanks for the comments both, I was being dim, and measuring from the wrong point.....

Now sorted, with the help of VarmLR! Much appreciated Paul!

Cheers

Chaz

 

Glad it's all sorted now

[Vortex]

Sorry to jump on a thread here but also a question on the COAL and distance to the Lands.

I have a measurement using the hornaday tool to measure of the ogive that gives me COAL of 2.185 or 2.720 to the tip of a 123gr lapua bullet.

 

Now here is my concern when I then measure the COAL to the lands using a modified case. Again measuring off the ogive I get a max COAL that my rifle can take without jamming it into the lands of 2.176. Again this is with a 123gr.

 

So that means my bullets are pushed into the lands by 0.009.

 

Does that chamber measurement seem short for a MAX COAL of only 2.176 touching the lands.

 

Regards,

Oliver

[snakeman]

As mentioned earlier I have no idea of the measurements for the 6.5's ...but if I were in your predicament I measure again the ogive in the modified case; and measure again...up to a dozen times and then take the average measurement following your tests... It could be your bullet might not be entering the neck square due to case run-out, or you could be misaligning the bullet causing you to have different readings - it's a starting point

[VarmLR]

Sorry to jump on a thread here but also a question on the COAL and distance to the Lands.

I have a measurement using the hornaday tool to measure of the ogive that gives me COAL of 2.185 or 2.720 to the tip of a 123gr lapua bullet.

 

Now here is my concern when I then measure the COAL to the lands using a modified case. Again measuring off the ogive I get a max COAL that my rifle can take without jamming it into the lands of 2.176. Again this is with a 123gr.

 

So that means my bullets are pushed into the lands by 0.009.

 

Does that chamber measurement seem short for a MAX COAL of only 2.176 touching the lands.

 

Regards,

Oliver

 

The difficulty in using different methods is in achieving any sort of consistency, which as Snakeman advises, is why it is a good idea to take multiple measurements and take an average.

 

Even if using the LNL OAL gauge I'll take 10 to 12 readings taking care only to use between 2 to 3 lbs pressure when feeding the bullet in, and also use a cleaning rod carefully inserted from the muzzle end with a plastic cap screwed to it to gently apply resistance from the other end. I can then insert a bullet and push gently into the lands and push back a few time until I feel the right amount of pressure to just kiss the lands. Measurements are usually within 4 or 5 thou max each time from each other with secant ogive bullets or less with tangent ogive designs and I just take an average,

 

I don't understand how you can use a modified case on its own if the Hornady one, as it has insufficient neck tension to grab the bullet enough, but using your own case which you have made from a fire formed case, it may simply be that you haven't enough neck tension and "grab" on the bullet so that it could be stopping slightly short of the lands. If you repeat a dozen times then check the average you may find that your comparisons are closer to each other.

[Vortex]

I understand that and have taken a few mesurements. What I was wondering if the COAL I am using is 2.185 and that is pushed into the lands, touching the lands gives COAL 2.176 if I wanted to give the bullet some jump my loaded round seems to be quite short compared to what it advises in the book of a minimum COAL to avoid too much pressure.

 

I don’t seem to have the length in the chamber to seat my bullets further out and any further back they would start to be seated too deep.

 

So does that mean the spec on the chamber could be out. I don’t have much room to play if you see what I mean compared with some that seat the bullet practically hanging out the case to get maximum case capacity.

[VarmLR]

Just looking at the .264 Lap Scenar 123gr comparing to the 139gr

 

Length is 1.303" compared with 1.358" for the 139gr.

 

Both seem to use the same/similar ogive profile which initially, I took to be a secant ogive for the 139 but looking at Litz data it's 9.7 calibres' radius ogive of 0.630" long, making it more a tangent ogive and (in my rifle) the COAL for the 139 is 2.872 inches.

 

The 123gr by contrast uses a longer 10.38 calibres' radiused ogive of 0.735" long and is closer to a more slippery secant profile but I'm unsure on this as it's form looks very similar to the tangent ogive of the 139.

 

Whatever the exact nature of the ogive, the thing that stands out here is that with a shorter overall length but longer radius ogive, the 123gr ought in theory sit a fair bit further out to touch the lands than the 139, not further in! (I'm guessing as I haven't tried any of the 123's yet that it may need to be more jump tolerant if sufficient neck bearing is to be retained and that may hint at it being more of a tangent or hybrid type profile).

 

This then seems to leave only two possibilities:

 

 

1). You have a very short throated barrel or

2) You may have measured your seating incorrectly.

 

 

It can only be one of the two things above.

[Laurie]

Looking back at my 6.5 Creedmoor notes for an out of the box factory rifle (not Tikka), the modified case-head to bullet ogive settings (nominally 15 thou' out) were 2.170" for the 123gn Scenar and 2.211" for the 139gn model. So, they don't appear to have the same nose. That gave actual COALs of 2.725 and 2.761 inches in this rifle. Note that the comparator readings are only usable as the name suggests for comparison / reference / setting and your Hornady 6.5mm insert is not at all guaranteed to have the same aperture size as mine. (Which is so old, it's a Stoney Point insert anyway, not Hornady)

 

The 136 Scenar L was the same as for the 139 give or take a couple of thou' suggesting a similar nose shape.

 

What I have found with the Creedmoor and I've seen references to this on the big 6.5mm Creedmoor forum is the possibility of undersized leades. If you look at the PT&G chamber drawing (which is the definitive one for the cartridge as PT&G worked with TC Arms and Hornady on the design), the leade diameter is specified at 0.2645", or a mere half-thou' above the nominal bullet diameter. Some bullets are slightly under the nominal 0.264", some slightly more, and one lot I've measured exceeded 0.2645"

 

The 260 Rem leade is also specified at this size (but not CIP 6.5X55mm SKAN which is a couple of thou' larger) and frankly I don't understand it. I can only think that this may go back to the 1980/90s when American 6.5mm bullets were usually undersize at 0.263-0.2635".

 

In any event, what it means is that if you get a rifle where the tolerances take it under the nominal leade size, and even with some bullets that match the leade inside diameter even if it is exactly as specified in the SAAMI chamber, the Hornady modified case / comparator readings can be all over the place as bullets can make a slight interference fit in the leade. In the case of the factory rifle I had (whose make I'm not going to specify) the Lapua Scenars were each and everyone 'tight'. My original comparator readings were as a result way too short as what appeared to be the bullet contacting the lands wasn't that at all. Chambering a proper length round and unloading showed mild scrape marks on the bullet shank from touching the leade walls.

 

Needless to say, the bullet touching the leade isn't just an issue over identifying COAL - it raises pressures and affects the rifle / ammunition performance. In testing, those bullets with slightly smaller diameters that slipped through the leade without touching it shot much better than those that were tight. The leade was rapidly eroded too by powder gases becoming rough at its entrance lip and presumably increasing pressure / bullet damage further.

 

As a further aside, the Stoney-Point / Hornady system is known to be only accurate to within a few thou' tolerances, so it is not wise to use it to set tiny jump figures, ie under 10 thou'. Moreover, I have found that you do not get even this level of precision when the rifle is custom built with a 'minimum SAAMI' gunsmith cut chamber, so I've not used it in 308 Win or 223 Rem for custom built FTR rifles for some years now for example. For a factory rifle in these and other cartridges, I'll use it happily as an easy to use and quick system.

[Vortex]

VarmLR Well it may well be a short throat, it’s a custom barrel on a 700 action. Measured a few times and my mesurements seem consistent and accurate.

 

Laurie when you say leade is that the same as free bore, if so that could be the isssue.

I get cratered primers but it’s a rem700 and did that when it was a .308. Huge hole the firing pin comes through.

I’m not getting any massive pressure issues and I’m up towards the top of the load data with 39.6gr of N150 with the 123gr bullet and it’s giving me approx 2970fps our of a 28inch barrel.

 

Wonder if there are slight “thinner” bullets that would avoid this tight leade.

 

It shoots well tho, little recoil and hits tennis balls at 600 so cant complain.

[Laurie]

The leade is the slightly over bullet diameter parallel section immediately ahead of the case-neck section of the chamber. Its length is in effect the freebore. The 'throat' is ahead of the leade and is the slightly tapered section where the lands are cut on an angle to provide an easy entry for the bullet, so I suppose it contributes marginally to the total freebore value.

 

The leade wouldn't normally have any rifling left in it at all as it should be marginally above bullet / groove diameter. P:T&G's standard 308 Win chamber is 0.310, ie 2 thou' greater than the nominal bullet and barrel groove diameters for example - but match chambers are specified closer to bullet diameters. My gunsmith friends also tell me that reamers as supplied usually cut a thou' above the nominal value as shown on the 'reamer print'.

 

If your rifle performs OK Vortex, you don't have a problem. As I said, an easy check though is to chamber then unload a round and see if there are signs of bullet to leade contact. (There will nearly always be a serious scrape mark at one point from the bullet touching something on the way in or out, but an over-tight leade for the bullet will produce slight marks around the entire shank circumference.) I have come across one Creedmoor rifle where the fit was so tight that I felt resistance on bolt closure and know of another (factory) example which had to be rechambered as factory rounds wouldn't chamber in it at all.

[Vortex]

I will chamber a few dummy rounds see what I get.

Just a bit of a puzzle as to why I can’t seat bullets further out before I hit the lands.

 

Cheers for all the advice guys.

[VarmLR]

That's some very useful and detailed info Laurie, thanks for sharing. I'd no idea that some Leades were made so tight or that they varied so much....ah the dangers of assuming anything in ballistics!! Mine is very slightly tight for a factory rifle to the extent that it wouldn't chamber my SST rounds at all (loaded to 30 thou off) and when I tested run-out on some of the dummy rounds loaded using a dial gauge I found that R/O was up to 0.008". Hardly precision but given that there's so much bullet forward of the neck it doesn't take much off-axis mis-alignment to result in that at the junction of the ogive to bullet main body. I thought "surely the Leade can't be that tight? It was. I dropped in a VLD stem to the seating die (Redding) and it seated the SSTs much more concentrically with an average of just 1 to 2 thou measured run-out at the same point and the rounds loaded easily. You live and learn.

 

I must admit though that the Hornady gauge readings weren't too spread out. The ES for 12 readings was 7 or 8 thou and the average somewhere (as you'd expect) in the middle.

 

Given even that variance plus allowing some tolerance variance on bullet manufacture, I rarely set closer than 15 thou for safety's sake and once I'd discovered how tight the leade was on mine, I have backed off to 30 thou.

[1234]

Anyone have a spare Hornady gauge as I am keen to have a play? Thanks

[1234]

Anyone have a spare Hornady gauge as I am keen to have a play? Thanks