|
Gunsmithing & How To Pro, Amateur & WECSOG and Tutorials, Guides & OLL Build Instructions |
|
Thread Tools | Display Modes |
#1
|
|||
|
|||
AR bolt hold open problem
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? |
#2
|
||||
|
||||
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.
__________________
Randall Rausch AR work: www.ar15barrels.com Bolt actions: www.700barrels.com Foreign Semi Autos: www.akbarrels.com Barrel, sight and trigger work on most pistols and shotguns. Most work performed while-you-wait, evening and saturday appointments available. |
#3
|
|||
|
|||
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 |
#4
|
|||
|
|||
What I saw in the books.
IMR 4895. Max 24.5 and website Mir 4895. 24.8C. (Compressed)
__________________
Been gone too long. It's been 15 to 20 years since i had to shelf my guns. Those early years sucked. I really miss the good old Pomona Gun Shows. I'm Back. |
#5
|
|||
|
|||
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? |
#6
|
|||
|
|||
Quote:
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. |
#7
|
||||
|
||||
Quote:
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.
__________________
Randall Rausch AR work: www.ar15barrels.com Bolt actions: www.700barrels.com Foreign Semi Autos: www.akbarrels.com Barrel, sight and trigger work on most pistols and shotguns. Most work performed while-you-wait, evening and saturday appointments available. |
#8
|
|||
|
|||
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. |
#9
|
|||
|
|||
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.
Sent from my iPad using Tapatalk |
#10
|
|||
|
|||
Quote:
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 |
#11
|
||||
|
||||
Quote:
Quote:
Quote:
Quote:
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. Code:
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! Code:
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!
__________________
Randall Rausch AR work: www.ar15barrels.com Bolt actions: www.700barrels.com Foreign Semi Autos: www.akbarrels.com Barrel, sight and trigger work on most pistols and shotguns. Most work performed while-you-wait, evening and saturday appointments available. Last edited by ar15barrels; 09-20-2018 at 11:04 PM.. |
#12
|
|||
|
|||
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. Last edited by Scota4570; 09-21-2018 at 7:43 AM.. |
#13
|
||||
|
||||
Quote:
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.
__________________
Randall Rausch AR work: www.ar15barrels.com Bolt actions: www.700barrels.com Foreign Semi Autos: www.akbarrels.com Barrel, sight and trigger work on most pistols and shotguns. Most work performed while-you-wait, evening and saturday appointments available. |
#14
|
|||
|
|||
Quote:
LC Military brass 100' above sea level 75* F ambient, the rifle was not shot rapidly, 1/ min. Thanks for doing this. |
#15
|
||||
|
||||
Quote:
Quote:
I think you have bad chrono data or your getting a dramatic velocity change from a hot barrel or something.
__________________
Randall Rausch AR work: www.ar15barrels.com Bolt actions: www.700barrels.com Foreign Semi Autos: www.akbarrels.com Barrel, sight and trigger work on most pistols and shotguns. Most work performed while-you-wait, evening and saturday appointments available. Last edited by ar15barrels; 09-21-2018 at 8:25 PM.. |
#16
|
||||
|
||||
H4895 is cut at longer stick length than Varget, are you doing charges by volume throw (and weighing after) or by trickle weight? Could make a difference, small volume with large stick size does not meter too well by volume.
Andrew - Lancaster, CA NRA Life Member, Calguns.net contributor, CGF / SAF / CRPA / FPC / USCCA member |
Thread Tools | |
Display Modes | |
|
|