AR bolt hold open problem

[Scota4570]

This is a nice match rifle. The bolt stays back with all factory ammo after the last shot.

With one hand load series, 69 gr Hornady and 21 thu 24 grain 4895 pull down powder with never holds open after the last shot. The top end loads are quite hot.

I also had the same problem with 846 pull down powder and 69 grainers.

Could it be that the bolt velocity is so fast that the bolt is bouncing back and missing the catch? Is the and issue with heavier bullets?

[ar15barrels]

How much distance do you have between the point where the bolt stop holds the bcg vs the absolute end of the bcg travel?
You need about 3/16" there for there to be enough TIME for the mag to pop the bolt catch up reliably.

If that distance is short, then slower vs faster operation of the BCG will effect the reliability of bolt stop actuation.

[Scota4570]

Thanks,

I'll measure it and see. If it is short should I take a little off the urethane bumper on the buffer?

I installed a DPQ right side hold open. It works 100% with all factory ammo. They extra mass may be an issue with hand loads? I was going to make another RH release but remove the unneeded material to make a minimal mass RH only release. Sound worthwhile?

S

[baih777]

What I saw in the books.
IMR 4895. Max 24.5
and website Mir 4895. 24.8C. (Compressed)

[Unbekannt]

Wouldn't difference in load influence everything? Let's start with gas pressure, then the speed the BCG moves, that influences the buffer/spring which is evidently insufficient to bounce back the BCG.

Why not just stick to ammo which works for you?

[Scota4570]

Wouldn't difference in load influence everything? Let's start with gas pressure, then the speed the BCG moves, that influences the buffer/spring which is evidently insufficient to bounce back the BCG.

Why not just stick to ammo which works for you?

I could never afford match grade ammo. I can load it and have plenty of components. These loads are shooting sub MOA with no particular fussing around in loading them. 24 gr is a top end load in my references for 69 grain bullets. I am surprised that the new data goes higher. 24 causes the ejector port to shave the case head 1 out of five. It is not wimpy. Other hand loads with lighter bullets work just fine.

Do the CMP guys make any modification the the buffer or spring for heavy bullets?

I made a new right hand only bolt release with no extra mass. I'll try it and see.

[ar15barrels]

24 gr is a top end load in my references for 69 grain bullets.
I am surprised that the new data goes higher.
24 causes the ejector port to shave the case head 1 out of five.
It is not wimpy.

Chamber pressure has little to do with gas port pressure.
Given equal chamber pressures, faster powders give you less gas port pressure.
Given equal chamber pressures, slower powders give you more gas port pressure.

Have you read my write up on AR gas system function?

You could be running a fast powder at high chamber pressures which sheds too much pressure by the time the bullet passes the gas port.

You could also be so over-gassed that the action is out of time and the bullet is not making it out of the barrel before the carrier tugs on the cartridge case.
The bullet needs to leave the barrel BEFORE the extractor tries to pop the cartridge free of the chamber.

Both of these are potential causes.

[Scota4570]

Just looked at the brass under magnification. The extractor is not distorting the rim or marking it badly, like if was pulling when the pressure was still high.

Fast powder? H4895 (mine is GIBRASS pull down) is right in the middle of the correct burning range. It is right next to Varget, a known good powder. I also tried WCC846(AKA BL-C2) with the same result. Wouldn't 23 grains being a good accurate load and 24 giving mild pressure signs confirms the appropriate burning rate?

Different weight buffers was new to me. Is there a way to determine if that is the issue without buying a bunch of buffers and randomly trying them? What should my rifle buffer weigh, for instance.

[scotty99]

AR bolt hold open problem

According to Hodgdon, 24grs is the STARTING load for H4895, and 26 is the max. A chrono would tell you for sure, but the simplest explanation is the load is too light.


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[Scota4570]

According to Hodgdon, 24grs is the STARTING load for H4895, and 26 is the max. A chrono would tell you for sure, but the simplest explanation is the load is too light.

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I removed one coil from the little coil spring on the bolt latch. I fabricated a minimal weight right hand only latch. It works 100% now. It works with only 21 grains 4895 (2250 fps). 24.6 grains gives 2750 fps with the 69gr bullet. The powder is behaving like IMR 4895. My scale is properly calibrated.

I think the extra mass of the ambidextrous latch was causing problems with some type of harmonic vibration with my hand-loads but not the factory ammo.

Since it works perfectly with the 21 grain load (2250 fps) I think the rifle is way "over gassed". I am going to experiment with an adjustable gas key and possibly an adjustable gas block. No need to beat the rifle up. I hear they shoot softer if a more moderate gas impulse is used.

Thanks All

[ar15barrels]

This is a nice match rifle.
With one hand load series, 69 gr Hornady and 21 thu 24 grain 4895 pull down powder with never holds open after the last shot.

24 gr is a top end load in my references for 69 grain bullets.

H4895 (mine is GIBRASS pull down) is right in the middle of the correct burning range.

I removed one coil from the little coil spring on the bolt latch.
I fabricated a minimal weight right hand only latch.
It works 100% now. It works with only 21 grains 4895 (2250 fps).
24.6 grains gives 2750 fps with the 69gr bullet.
The powder is behaving like IMR 4895.

I need more details on your setup.
Hornady does not sell a 69gr bullet.
I'll assume you are using the 68gr Hornady part number 2278.
You stated this is a match rifle, so I'll assume 20" because that's what most match rifles will use but your stated velocities seem like you have as much shorter barrel.

Cartridge          : .223 Rem. (SAAMI)
Bullet             : .224, 68, Hornady BTHP M 2278
Useable Case Capaci: 24.411 grain H2O = 1.585 cm³
Cartridge O.A.L. L6: 2.260 inch = 57.40 mm
Barrel Length      : 20.0 inch = 508.0 mm
Powder             : IMR 4895

Predicted data by increasing and decreasing the given charge,
incremented in steps of 2.084% of nominal charge.
CAUTION: Figures exceed maximum and minimum recommended loads !

Step    Fill. Charge   Vel.  Energy   Pmax   Pmuz  Prop.Burnt B_Time
 %       %    Grains   fps   ft.lbs    psi    psi      %        ms

-20.8   84    19.00   2258     770   26788   6602     81.8    1.311
-18.8   87    19.50   2320     812   28710   6853     83.3    1.274
-16.7   89    20.00   2381     856   30778   7101     84.9    1.238
-14.6   91    20.50   2444     902   33006   7345     86.3    1.204
-12.5   93    21.00   2506     948   35406   7582     87.7    1.170
-10.4   96    21.50   2569     996   37995   7813     89.1    1.135
-08.3   98    22.00   2632    1046   40790   8036     90.4    1.099
-06.3  100    22.50   2695    1097   43810   8251     91.6    1.064
-04.2  102    23.00   2759    1149   47065   8456     92.7    1.031  ! Near Maximum !
-02.1  104    23.50   2822    1203   50551   8650     93.8    0.998  ! Near Maximum !
+00.0  107    24.00   2886    1257   54310   8833     94.8    0.967  !DANGEROUS LOAD-DO NOT USE!
+02.1  109    24.50   2950    1314   58376   9003     95.7    0.937  !DANGEROUS LOAD-DO NOT USE!
+04.2  111    25.00   3014    1371   62781   9160     96.5    0.908  !DANGEROUS LOAD-DO NOT USE!
+06.3  113    25.50   3078    1430   67558   9303     97.3    0.880  !DANGEROUS LOAD-DO NOT USE!
+08.3  116    26.00   3142    1490   72750   9429     98.0    0.853  !DANGEROUS LOAD-DO NOT USE!
+10.4  118    26.50   3206    1551   78403   9539     98.5    0.827  !DANGEROUS LOAD-DO NOT USE!

Any chance this is actually a 14,5" barrel?

Cartridge          : .223 Rem. (SAAMI)
Bullet             : .224, 68, Hornady BTHP M 2278
Useable Case Capaci: 24.411 grain H2O = 1.585 cm³
Cartridge O.A.L. L6: 2.260 inch = 57.40 mm
Barrel Length      : 14.5 inch = 368.3 mm
Powder             : IMR 4895

Predicted data by increasing and decreasing the given charge,
incremented in steps of 2.084% of nominal charge.
CAUTION: Figures exceed maximum and minimum recommended loads !

Step    Fill. Charge   Vel.  Energy   Pmax   Pmuz  Prop.Burnt B_Time
 %       %    Grains   fps   ft.lbs    psi    psi      %        ms

-20.8   84    19.00   2054     637   26788   8948     76.6    1.099
-18.8   87    19.50   2112     674   28710   9330     78.3    1.068
-16.7   89    20.00   2171     712   30778   9710     79.9    1.038
-14.6   91    20.50   2231     751   33006  10087     81.6    1.008
-12.5   93    21.00   2291     793   35406  10461     83.1    0.979
-10.4   96    21.50   2352     835   37995  10830     84.7    0.949
-08.3   98    22.00   2414     880   40790  11192     86.1    0.918
-06.3  100    22.50   2476     926   43810  11546     87.6    0.887
-04.2  102    23.00   2538     973   47065  11891     88.9    0.858  ! Near Maximum !
-02.1  104    23.50   2601    1022   50551  12224     90.2    0.830  ! Near Maximum !
+00.0  107    24.00   2665    1072   54310  12545     91.5    0.802  !DANGEROUS LOAD-DO NOT USE!
+02.1  109    24.50   2729    1124   58376  12852     92.7    0.776  !DANGEROUS LOAD-DO NOT USE!
+04.2  111    25.00   2793    1178   62781  13142     93.8    0.750  !DANGEROUS LOAD-DO NOT USE!
+06.3  113    25.50   2858    1233   67558  13414     94.8    0.726  !DANGEROUS LOAD-DO NOT USE!
+08.3  116    26.00   2923    1290   72750  13668     95.7    0.702  !DANGEROUS LOAD-DO NOT USE!
+10.4  118    26.50   2988    1348   78403  13900     96.6    0.679  !DANGEROUS LOAD-DO NOT USE!

[Scota4570]

Randall,
Yes, 68 grain.
Yes, 20" BBL

The velocity is low per the tables. Could be a chamber or throat variation. It is shooting very well, sub MOA. I only shoot to 300 yards so I am not super concerned over 150 FPS if the accuracy is good.

I bet the ejector hole shear mark on the brass is from a burr on the edge of the hole. I'll examine it carefully and check. It seems to give a false high pressure indication.

Any thoughts on adjustable gas keys vs adjustable gas blocks? How about adding a spring between the weights in the buffer? The idea is smoother functioning and less wear on the rifle.

At any rate the function problem is solved with the bolt latch. That will get me through the next match.

[ar15barrels]

The velocity is low per the tables.
Could be a chamber or throat variation.

The tables are based on standard test conditions.
Low velocity means low pressure as velocity in a given barrel length absolutely tracks with pressure.

Case capacity and throat length and temperature all effect pressure.
What temperature was the ammo/rifle when you were chrongraphing?

Measure your actual case capacity in grains of water from 3 fired cases that have not been deprimed or resized yet and let me know what you get.
Make sure to tap on the cases and get ALL the micro air bubbles out.
Make sure the water is flat at the case mouth.

I can adjust the model to match your actual test results and then we will have a more reliable idea what chamber pressures you are actually running.

[Scota4570]

The tables are based on standard test conditions.
Low velocity means low pressure as velocity in a given barrel length absolutely tracks with pressure.

Case capacity and throat length and temperature all effect pressure.
What temperature was the ammo/rifle when you were chrongraphing?

Measure your actual case capacity in grains of water from 3 fired cases that have not been deprimed or resized yet and let me know what you get.
Make sure to tap on the cases and get ALL the micro air bubbles out.
Make sure the water is flat at the case mouth.

I can adjust the model to match your actual test results and then we will have a more reliable idea what chamber pressures you are actually running.

30.6 grains full up. 28.5 to the shoulder neck junction.
LC Military brass

100' above sea level

75* F ambient, the rifle was not shot rapidly, 1/ min.

Thanks for doing this.

[ar15barrels]

21 grains 4895 (2250 fps).
24.6 grains gives 2750 fps with the 69gr bullet.

30.6 grains full up. 28.5 to the shoulder neck junction.
LC Military brass

100' above sea level

75* F ambient, the rifle was not shot rapidly, 1/ min.

Thanks for doing this.

I can't get your velocities to jive with your charge weights at 75 degrees with 30.6gr case capacity without changing the burning rate of the powder, but then the other charge weight will not match up with the velocity you gave.
I think you have bad chrono data or your getting a dramatic velocity change from a hot barrel or something.