Complex sentence: This was because the RAF's Brownings had been modified to fire from an open bolt; while this prevented overheating of the cordite used in British ammunition, it allowed cold air to flow through the barrel unhindered.
Split into simpler sentences: This was because the RAF's Brownings had been modified to fire from an open bolt. While this prevented overheating of the cordite used in British ammunition, it allowed cold air to flow through the barrel unhindered.