If you chamber a pellet with an undersized skirt, will the initial burst of air expand the skirt to engage the barrel?
Pellet Skirt
- Thread starter easyridemm
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This is my opinion, and so far as I know, unproven.
The answer is, it depends, If the skirt is close to the correct dimensions, yes it will expand, If too small or heavily deformed, the pellet skirt may not fully expand, or more air may rush past it before it completes expanding, impacting power and accuracy.
So expanding skirts, yes it does happen, but how much air gets past before the skirt expands is the wild card, and that has real effects.
So, if shooting for plinking - it's kinda , "Who cares, just shoot it". If shooting for accuracy, it makes a difference.
The answer is, it depends, If the skirt is close to the correct dimensions, yes it will expand, If too small or heavily deformed, the pellet skirt may not fully expand, or more air may rush past it before it completes expanding, impacting power and accuracy.
So expanding skirts, yes it does happen, but how much air gets past before the skirt expands is the wild card, and that has real effects.
So, if shooting for plinking - it's kinda , "Who cares, just shoot it". If shooting for accuracy, it makes a difference.
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I only ask this as I am trying to understand why you would want to “roll” your pellets? Excluding deformed pellets, if the skirt is undersized and will expand, and certainly the barrel will size an oversized skirt, what is the point in “rolling”. Just curious
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Some skirts certainly could expand in the right conditions. The best conditions for expansion would include thin-walled skirts with enough burst/blast of air behind them that lean toward being made with pure/soft lead with very little antimony in them, like JSBs, Air Arms or FX branded pellets. Probably could throw in deep dish bases as well. Also, how much grab the lands have holding the pellet back, too. These would be the best scenarios for skirt expansion to occur. So, a lot of variables in-between all of this that are also possible. The question remaining would be how consistent the string would be, given all of these variables. Not worth the gamble if a serious air gunner shooting at low powers using hard lead pellets with thicker skirts.
With all of this said, I size all of my pellets that are .25 caliber and up. This would include the skirts in the process. I have had no issues with getting very tight ES strings or consistently tight groups at distance. But, I use soft lead and run at high FPE with mine and all heads and skirts are sized the same.
With all of this said, I size all of my pellets that are .25 caliber and up. This would include the skirts in the process. I have had no issues with getting very tight ES strings or consistently tight groups at distance. But, I use soft lead and run at high FPE with mine and all heads and skirts are sized the same.
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Let's assume that the skirt WILL expand due to the air charge. Seems some of the air charge would leak by before expansion could lead to sealing. Since there is a fixed amount of air charge, it seems even if they would "expand", there could be shot variance related to any such leakage. JMO and as noted above, hard to prove either way.
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It may expand the skirt or it may not. Depends on the pellet and caliber some have much thicker skirts than others. Also depends on the gun some guns are anemic power wise so are not going to expand any skirt much. Others may do well with expanding many skirts.
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If you have sorted your pellets for head size then rolling gives you the head to skirt difference.
You roll your pellets and pick the group or groups that shoot the best. It's a simple thing most guns will shoot one or two of the different roll sorted groups a bit better than the others. It's like fine tuning your sorting to a higher degree.
It also lets you discern a damaged skirt on a pellet BEFORE you have a flyer.
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Rolling pellets involves first indexing the pellet to some point along a line. This indexing process "fixes" the angle of the pellet with respect to the "starting" line and defines the center of the rolling radius. Said another way. When you index the pellet you are putting a cone against a line at a specific point. Once you have done that, when rolled, that cone will describe a curve around a point which is the center of that rolling radius. That curve IS NOT a function of the ratio of the head diameter to the skirt diameter. It is a function of the specific head diameter, skirt diameter, and distance between those two bearing surfaces.
There is a proof of this which has been extensively discussed here:
Geometric Proof AGG Discussion
and here:
Geometric Proof AGN Discussion
Regarding your specific question:
Rolling checks those three parameters, namely the head diameter, the skirt diameter, and the distance between those two bearing surfaces. As such it serves to group pellets which are similar in those three parameters together. It does not test a number of other parameters such as, but not limited to weight, skirt thickness, over all length, or damage which does not appear in one of those mentioned parameters.
There are a number of topics and empirical threads regarding the improvement (which has been measured) that supports the conclusions established in the above proofs. They can be found in the posts linked here:
Part One
Part Two
Part Three
Part Four
Part Five
As with any thing you do to try to sort pellets there are advantages and disadvantages. The advantage is tighter groups. The disadvantages are not so obvious. Typically when sorting pellets you might find that you throw out as many as 20 or 25 percent of a batch of pellets as culls. These are pellets which would enlarge your groups. That makes every pellet you do actually shoot more expensive both in time and money. It does, after all, take time to roll pellets, just as any sorting method takes time. That time could be put to use learning how to better judge wind (for example). It may therefore be that your time can be better spent OR NOT. That is a personal call, a choice. Some will choose to make that choice, some will not.
As a hunter, my ethic demands I humanely dispatch my prey. If I can reduce the diameter of my groups by 20% at some distance then I am going to make that choice. Another person may not decide to sort pellets. That person may decide to simply pass up shots (as too long) that I may elect to take. Again, it is a PERSONAL choice.
Many bench rest shooters believe rolling pellets is a waste of their time BECAUSE wind is the single largest introducer of error in their groups. The rifles easily shoot half or three fourths MOA but wind OFTEN enlarges a match winning group to easily three times that. Clearly it would be exceptionally difficult to PROVE rolling pellets helped a shooter to win in any specific match. Again it is a PERSONAL choice.
When I want to shoot groups that look like this:
Personal Best
I roll pellets.
There is a proof of this which has been extensively discussed here:
Geometric Proof AGG Discussion
and here:
Geometric Proof AGN Discussion
Regarding your specific question:
Rolling checks those three parameters, namely the head diameter, the skirt diameter, and the distance between those two bearing surfaces. As such it serves to group pellets which are similar in those three parameters together. It does not test a number of other parameters such as, but not limited to weight, skirt thickness, over all length, or damage which does not appear in one of those mentioned parameters.
There are a number of topics and empirical threads regarding the improvement (which has been measured) that supports the conclusions established in the above proofs. They can be found in the posts linked here:
Part One
Part Two
Part Three
Part Four
Part Five
As with any thing you do to try to sort pellets there are advantages and disadvantages. The advantage is tighter groups. The disadvantages are not so obvious. Typically when sorting pellets you might find that you throw out as many as 20 or 25 percent of a batch of pellets as culls. These are pellets which would enlarge your groups. That makes every pellet you do actually shoot more expensive both in time and money. It does, after all, take time to roll pellets, just as any sorting method takes time. That time could be put to use learning how to better judge wind (for example). It may therefore be that your time can be better spent OR NOT. That is a personal call, a choice. Some will choose to make that choice, some will not.
As a hunter, my ethic demands I humanely dispatch my prey. If I can reduce the diameter of my groups by 20% at some distance then I am going to make that choice. Another person may not decide to sort pellets. That person may decide to simply pass up shots (as too long) that I may elect to take. Again, it is a PERSONAL choice.
Many bench rest shooters believe rolling pellets is a waste of their time BECAUSE wind is the single largest introducer of error in their groups. The rifles easily shoot half or three fourths MOA but wind OFTEN enlarges a match winning group to easily three times that. Clearly it would be exceptionally difficult to PROVE rolling pellets helped a shooter to win in any specific match. Again it is a PERSONAL choice.
When I want to shoot groups that look like this:
Personal Best
I roll pellets.
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"Many bench rest shooters believe rolling pellets is a waste of their time BECAUSE wind is the single largest introducer of error in their groups."
Seems that the wind wouldn't know rolled from not. I'd say a pellet that COULD produce a smaller group (by virtue of rolling or weighing or head sizing or any other type of sorting) would be starting at a better baseline from which the wind would do its thing equally to sorted and unsorted and thus sorted should shoot smaller groups. Even though wind does seem to be the main source of error when present.
Seems that the wind wouldn't know rolled from not. I'd say a pellet that COULD produce a smaller group (by virtue of rolling or weighing or head sizing or any other type of sorting) would be starting at a better baseline from which the wind would do its thing equally to sorted and unsorted and thus sorted should shoot smaller groups. Even though wind does seem to be the main source of error when present.
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Some examples of skirt expansion on firing...
Courtesy of rsterne. The two on the left are fired and captured, two on the right were pushed through the barrel. Note the ring at the base of the skirt showing contact with the bore:
Courtesy of Ribbonstone. This pellet got smacked pretty hard in the rear end to get up to 1040fps but the obturation is still impressive considering how thick the EunJin skirts are:
And a side-by-side showing the increase in bearing surface resulting from being fired at a higher velocity.
Courtesy of rsterne. The two on the left are fired and captured, two on the right were pushed through the barrel. Note the ring at the base of the skirt showing contact with the bore:
Courtesy of Ribbonstone. This pellet got smacked pretty hard in the rear end to get up to 1040fps but the obturation is still impressive considering how thick the EunJin skirts are:
And a side-by-side showing the increase in bearing surface resulting from being fired at a higher velocity.
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What a great bunch of information on an intriguing subject. Good feedback as always. Thank You
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Some examples of skirt expansion on firing...
Courtesy of rsterne. The two on the left are fired and captured, two on the right were pushed through the barrel. Note the ring at the base of the skirt showing contact with the bore:
Courtesy of Ribbonstone. This pellet got smacked pretty hard in the rear end to get up to 1040fps but the obturation is still impressive considering how thick the EunJin skirts are:
And a side-by-side showing the increase in bearing surface resulting from being fired at a higher velocity.
Not sure I'd say without question that the first picture illustrates skirt expansion. Consider a flared skirt pellet pushed into a bore. Head engages rifling as the picture shows and if the skirt is "flared" (wider than head) then what is shown could simply be the resulting contact with the bore (wider skirt being squeezed down by bore). But I do bet expansion does occur.
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As described in the original post, the two pellets to the right were pushed through the same bore and do not show have the same degree of engagement. The larger contact patch of the fired pellets on the left is a direct result of obturation.
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I missed the "pushed through" part. When I looked at the photo, it appeared there were rifling marks on the heads of the left two and not on the right two. Probably just bad eyes. As described, it does seem to support expansion.
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