Crimping and muzzle vel SD? 7.62 Nato, long free jump

[jtlassila99]

Onko kovia mielipiteita crimp vs no crimp?   7.62 Nato

Sopiiko jos kirjoittelen englanniksi?

My test results below made me order a Lee Factory crimp die for 308Win.

I have been doing load development with 7.62 Nato, Remington 700P.   Athlon radar.

I normally load SMK168, GGG Nato case, 41.5gns Varget, WLR-M primer.  This produces 2600fps avg, 9-12fps SD and shoots pretty good.

I loaded some Hornady 168 match.  It averaged the same 2600fps but has 30+ fps SD.  ??

I compared the two bullets; The SMKs mic 0.3084.  The Hornadys mic .3081.  A tiny difference.

I had previously tried to check free jump with the SMK168 using the cleaning rod method and found free jump to be about 0.095".  However, I never did see rifling marks on the bullet, just scuffing all around.  Appears the test bullet was not all the way to the lands.

So, I chucked a SMK in a drill and turned the bullet down to 0.3070 with some sand paper.  this slimmer bullet goes into the throat and all the way to the rifling lands, now leaving land marks.  The test shows the lands are far away;  The free jump measures .650" !!

So, I think the fatter SMK bullet gets stuck in the cone whereas the skinnier HRN bullet pops out the case upon firing, all the way to the lands resulting in poor velocity SD.

I have seen this same thing in Finn Mosins, where crimping greatly reduces vel SD w/o hurting accuracy.

What thoughts, opinions, suggestions, might you have?


A few pictures below.  This testing was shot quickly in fading light, for velocity, not for group size.   Please, ignore the .223 holes.

[MK]

En tiedä tuon Remarin ylimenon pituudesta mutta Mosinin D-pesässä ei saa rihlakosketusta tehtyä laisinkaan VLD- luodeilla. Crimppauksen vaikutus on nostaa pesäpainetta ja parantaa syttymistä jos painetta ei saa tehtyä rihlakosketuksella. Käytännössä taitaa olla vaikea tehdä tuosta crimpistä niin tasaista että se antaisi saman SDn kuin lyhyt ylimeno ja rihlakosketus. Mosininilla on tullut kisailtua perinneaseissa jo useampi vuosikymmen, ratkaisin tuon pitkä ylimenon ongelmat lataamalla patruunat niin lyhyiksi kuin mahdollista. SD paranee kun painetta saa nostettua pienemmällä ruutitilavuudella ja käyttämällä luodin massaa hyväksi paineen nousuun. QL laskenta ja lähtönopeusmuutos tukee lyhyen latauspituuden teoriaa ja myös tulokset taulussa. 

[KJJ]

ootko kokeillu lapuan 220 grain scenua muistaakseni ei saanut niin pitkäksi ladattua kuin bergereitä 300LM

ps ajoainettahan voi vaihtaa nopeenpaan tosin saako sillä sitten sattuun..

[HJu]

Krimppi kuluttaa hylsyä. En käytä pulttilukoissa.

[rkr]

If you want accurate measurements for COAL you should make one of these:

IMG20260401085756 by abbababbaccc, on Flickr

Dremel with cutting wheel and for slits in a case. Push in bullet so it's well longer than your estimated COAL, feed it in closing the bolt, carefully draw it out and measure what's your land contact COAL with the bullet you are using.

If you need to reduce the bullet diameter to get land contact I would guess you have a nasty carbon ring in your chamber.

[jtlassila99]

If you want accurate measurements for COAL you should make one of these:

Dremel with cutting wheel and for slits in a case. Push in bullet so it's well longer than your estimated COAL, feed it in closing the bolt, carefully draw it out and measure what's your land contact COAL with the bullet you are using.

If you need to reduce the bullet diameter to get land contact I would guess you have a nasty carbon ring in your chamber.

I do use the same method when applicable.  I just take a fired case and shrink the neck to just loosely hold the bullet. 

I am not sure if you quite understood the purpose of my post.  I made the illustration below to try to visualize it.

The lands in my R700P are actually very far away, the free is jump about .650", or 17mm. Disappointing.

However, the SMK, being fat, gets stuck in the tapered cone and shows apparent free jump of about 0.100".  The Hornady is much closer to .3080" and barely makes contact with the cone with inconsistent free jump measurement.  The Hornady loads showed high velocity SD and poor grouping.

A turned down .3070 test bullet clearly shows how far the lands are, about .650" away, with my typical loaded round.

If you want to know for sure what your real free jump is, test with a bullet turned down to just below nominal OD, like .3070.  The lands are 0.004" for a .300" bore.

A chamber like mine is very sensitive to the slightest variation in bullet OD as skinnier bullets will pop out of the case all the way to the lands when the primer goes off.

Test your chamber like this and post the results, please.

[HJu]

Now that you know that Hornady bullet is too thin and SMK is about right for your rifle I would make the decision to drop Hornady from further load development.

[jtlassila99]

Now that you know that Hornady bullet is too thin and SMK is about right for your rifle I would make the decision to drop Hornady from further load development.

It may not be that simple.  I'm seeing tiny variations in bullet OD from one lot to another.  I have SMK168s that OD .3084 and I have some older ones that measure .3082.  My latest box of HRN168 ODs .3081, but I have some older HRN168 reloads that show .3083 bullet OD at case mouth.

My testing shows that when the bullet OD at case mouth is > .3082, the bolt tends to close hard and the bullet has scuff marks if the round is ejected.  If the bolt closes really hard, then the bullet will get stuck and will not eject, spills powder. 

When the bolt closes hard, vel SD is good, like 6-9fps.
 
When bullet OD at case mouth is < .3082, the bolt tends to close easy and velocity SD is poor, whether SMK or HRN.

The GGG factory Nato ammo always has great velococity SD, lowest of all tested.  It has a heavy crimp on the  case neck to retain the bullet.

How many .308 chambers are like this R700?  In this chamber, the lands are far away and the tiniest variations in bullet OD determine apparent free jump and affect vel SD.   

Check you chamber and post results here, please.

[HJu]

What kind of measurement  tools you have if you can measure up to 0.0001" accuracy? In millimiters that is 0,00254mm while a regular micrometer can measure only with 0.01mm accuracy.

Typical factory rifle throats tend to be longish to keep pressures at lower levels. If you want a blueprinted chamber and throat the order a Shilen Select match barrel and let a benchrest gunsmith chamber it. Still different bullet ODs will change pressures because the barrel ID is fixed.

[jtlassila99]

What kind of measurement  tools you have if you can measure up to 0.0001" accuracy? In millimiters that is 0,00254mm while a regular micrometer can measure only with 0.01mm accuracy.

Typical factory rifle throats tend to be longish to keep pressures at lower levels. If you want a blueprinted chamber and throat the order a Shilen Select match barrel and let a benchrest gunsmith chamber it. Still different bullet ODs will change pressures because the barrel ID is fixed.

I have a regular, cheap micrometer.  It measures to 0.0001.   

I am kind of disappointed in the this R700 chamber.  I do wonder how common this is in mass produced .308 guns.  I do kind of want to get a long distance rifle with a match barrel.

If you have a .308, test for how far the lands are using a .3070 bullet or rod of some kind.

[HJu]

If the rifle is not a match rifle it very probably have a loose chamber and throat. Sakos and Tikkas tend to have better than average chambers and throats.

I would concenteate on the groups and not so much on chambers. If you get better groups with crimping then by all means use it.

[jtlassila99]

If the rifle is not a match rifle it very probably have a loose chamber and throat. Sakos and Tikkas tend to have better than average chambers and throats.

I would concenteate on the groups and not so much on chambers. If you get better grouos with crimping then by all means use it.

I do not like that the bolt closes hard on some of my loads.  I'd rather have some free jump.  I have used crimping in M39 loads to get SD to under 10fps without hurting accuracy.

I have a .308 finish reamer that I used to headspace my M14 clone builds.  I'm thinking of using it to touch up the throat. 

[jtlassila99]

So, I went ahead and used the head spacing finish reamer.  Interestingly, it did get tight right away.  It started to cut well before the bolt was closed.  It did cut where I expected, in the throat, but also cut a little in the neck.

I first checked head space. It was Go + 1 x scotch tape (+0.001").  After reaming, the head space checked the same, no change, which is what I wanted.

With this reamer, the bolt needs to be stripped and one turns the reamer with a vise-grip on the rod at the muzzle while slowly closing the bolt.

Afterwards, I checked with several bullets.  They all now seem to show free jump of about 0.100"  Not ideal perhaps, but better than stuck bullets and spilled powder upon extraction of a chambered round and needing a cleaning rod to pound the stuck bullet out.

[jtlassila99]

I did some Teslong borescope work on my R700P.   The video shows my finish reamer cut where I expected it to cut, in the throat just outside the case mouth.   But it also shows cutting in the neck, which was unexpected to me.

Now I'm thinking some of my hard closing bolt was due to the neck getting pinched rather than the bullet being tight in the throat.

Regardless, now I measure consistent free jump of .150" to the lands. 




Google album with bore cam videos below. 

R700P bore cam videos Google Album here...

The videos start about an inch or so down the barrel from the throat, moving towards the breach.   You can see where the lands taper off in the throat.  Then you can see the fresh metal where the reamer cut the throat just outside the case mouth.  The step for the case mouth is clearly visible in the chamber.  The the reamer cut quite a bit in the neck, seemingly the entire length of the neck.

The GGG Nato cases have thick necks measuring .3360-.3370 with my typical .308 bullets.  The Saami spec for the neck is .3400.  I'm thinking this R700 chamber had a tight neck and throat.