300BlkTalk

Here are the first results again (new image hosting platform)

The bolt closes with minimal effort at 2.14" (54.5mm). These loads are 2.08" (53mm). The jump to lands was .06" although given the inconsistency of measured bullets some jumps were closer to .04".




Note the right hand column in the spreadsheet are 220SMK's. This is my normal sub load and in this case acts as a default control in the test. The SMK's were fine and pencilled through the target as you would expect.

The speed of sound at the time was 1,140fps.

My theory is that that the bullet's relatively large void is prone to instability. The large void lends itself to instability if spun and the walls are not exactly concentric. The faster the spin the sooner the wobble deteriorates until it is tumbling at 100yards.

I double checked the AR barrel, and apparently my calipers were off. Measured three times, all at 2.115 oal in a "I don't recall" brand barrel. Most likely, Anderson. I also checked an 18" McGowen AR barrel, that measured 2.088 oal.
CBTO in the Anderson- 1.723
McGowen AR barrel- 1.693
All I have on hand is a Hornady 225 bthp that measured:
2.335 oal - Anderson
2.318 oal - McGowen

Why do I have this feeling I'm going to be buying a new barrel for the 700

ozleux wrote:I double checked the AR barrel, and apparently my calipers were off. Measured three times, all at 2.115 oal in a "I don't recall" brand barrel. Most likely, Anderson. I also checked an 18" McGowen AR barrel, that measured 2.088 oal.
CBTO in the Anderson- 1.723
McGowen- 1.693
All I have on hand is a Hornady 225 bthp that measured:
2.335 oal - Anderson
2.318 oal - McGowen

Why do I have this feeling I'm going to be buying a new barrel for the 700

ozleux wrote:2.125 in an AR
2.380 in McGowen 700 barrel

Measuring while drunk

How is the Mcgowen shorter .017" CTBO, but longer .255" COL?

Klem wrote:snip

Here are the first results again (new image hosting platform)

The bolt closes with minimal effort at 2.14" (54.5mm). These loads are 2.08" (53mm).

snip

Note the right hand column in the spreadsheet are 220SMK's. This is my normal sub load and in this case acts as a default control in the test. The SMK's were fine and pencilled through the target as you would expect.

My theory is that that the bullet's relatively large void is prone to instability. The large void lends itself to instability if spun and the walls are not exactly concentric. The faster the spin the sooner the wobble deteriorates until it is tumbling at 100yards.

That's why I was wondering how easy it is to damage the tip.

I found one reference to a guy that filled a seating stem with hot glue to make a custom seater that fit the bullet. Problem solved for him.

dellet wrote:

ozleux wrote:I double checked the AR barrel, and apparently my calipers were off. Measured three times, all at 2.115 oal in a "I don't recall" brand barrel. Most likely, Anderson. I also checked an 18" McGowen AR barrel, that measured 2.088 oal.
CBTO in the Anderson- 1.723
McGowen- 1.693
All I have on hand is a Hornady 225 bthp that measured:
2.335 oal - Anderson
2.318 oal - McGowen

Why do I have this feeling I'm going to be buying a new barrel for the 700

ozleux wrote:2.125 in an AR
2.380 in McGowen 700 barrel

Measuring while drunk

How is the Mcgowen shorter .017" CTBO, but longer .255" COL?

I have 2 McGowen barrels, one on a 700, on in an AR. I got the second measurements from the AR. I'm starting to wish I was drunk. I've got a feeling the chamber in the 700 may not be exactly right.

Damage to the tip is one theory. The other is a lack of concentricity from the factory (exacerbated by fast twist barrels).

I don't see any visible damage to the tips from my Forster seating die. That said I have backed out the body and screwed in the seater to compensate. I'm not ruling anything out given I have 150 of these things and have only fired about 30.

I'll let you know what happens.

I know these things are expensive, but does anyone that has them have the ability to cross section one? Tip to tail. I would be interested to see how far the lead runs to base, if the base is all copper and that hollow cavity. There has been enough trouble with this bullet that I am leery of buying some, however if someone is at their wits end with this thing, I'll fool with a box this summer and try and sort it out. Dellet has more twist options than I do, but I believe the answer lies in it's makeup. Might be wrong though.

Here's the next instalment in the 220NBT saga...another 30 rds unstable at the target 100 yard away.

This is ten shots bench-rested using a red dot scope at 100yds with no suppressor. A 6-MOA group tumbling through the target. Velocity; 1,030fps.

I unscrewed the die and screwed in the seating stem of the Forster to make sure there was no chance the die body was damaging the bullet. There were no marks on the loaded bullet (like the first batch).

I'm scratching my head for answers.

Have any other sub rounds you can try to see if they are doing the same thing? the hornady 190 subs are out now so should be easy to get some. I know your hand loading these but a commercial round could help figure this out if your barrel is bad.

bearcatrp wrote:Have any other sub rounds you can try to see if they are doing the same thing? the hornady 190 subs are out now so should be easy to get some. I know your hand loading these but a commercial round could help figure this out if your barrel is bad.

As I said earlier in the thread, my usual sub load is the 220SMK and this was shooting fine on the day. I have been shooting Blackout since 2011 and trialled a number of heavy bullets for subsonic (220SMK, 240SMK, 220Sierra RN, 220Hornady RN, 210Berger, 220Outlaw State SN, 220 Blackout Bullets HP, the 200Lapua and a few light bullets with Trail Boss for non-cycling super-quiet). This is the first time I have seen any go unstable.

Nosler replied again saying they have seen this instability on two occasions;

1. When a muzzle device interferes with the gasses surrounding the bullet on exiting.
2. When slow powder does the same.

I have been using H4198 but will try a faster powder.

Anyone here used these and had them come out stable using a powder that cycles the action?