Our footwear, apparel, optics, electronics, and accessories, camping, fishing, and hunting and shooting departments saw increases of $4.0 million, $3.0 million, $2.2 million, $1.7 million, $0.9 million and $0.2 million, respectively, in the third quarter of fiscal year 2021 compared to the comparable 13-week period of fiscal year 2020 due to the opening of seven new stores since October 31, 2020. Within the hunting and shooting department, our firearm category saw a decrease of $9.3 million or 10.0%, while our ammunition category saw an increase of $1.8 million or 2.9% in the third quarter of fiscal year 2021 compared to the comparable 13-week period of fiscal year 2020. The decrease seen in the firearm category is primarily due to lower demand as we anniversaried the social unrest and pending presidential election of the prior year period. The increase in the ammunition category is due to the opening of seven new stores since October 31, 2020, partially offset by supply chain disruptions.
With respect to same store sales, during the 13 weeks ended October 30, 2021, our footwear, apparel, and optics, electronics, and accessories, departments had increases of 14.3%, 5.6%, and 3.5%, respectively, compared to the comparable 13-week period of fiscal year 2020. Our hunting and shooting, fishing, and camping departments saw decreases of 6.1%, 1.8%, and 0.4%, respectively, as we anniversaried the demand driven in the prior year by the COVID-19 pandemic, social unrest, and the pending presidential election. As of October 30, 2021, we had 109 stores included in our same store sales calculation.
Gross Profit. Gross profit decreased to $129.6 million during the 13 weeks ended October 30, 2021 compared to $130.6 million for the corresponding period of fiscal year 2020. As a percentage of net sales, gross profit decreased to 32.3% for the 13 weeks ended October 30, 2021, compared to 33.9% for the corresponding period of fiscal year 2020 due to higher freight costs. The higher freight costs were partially offset by an increase in product margins across most departments and increased vendor incentives. We expect higher transportation costs to continue to impact the business during the remainder of fiscal 2021.
Selling, General, and Administrative Expenses. Selling, general, and administrative expenses increased by $7.8 million, or 8.4%, to $100.0 million during the 13 weeks ended October 30, 2021 from $92.2 million for the comparable 13-week period of fiscal year 2020. This increase was primarily due to an increase in our payroll expense of $2.8 million, which was the result of opening seven new stores since October 31, 2020, and minimum wage increases impacting 12 of our stores in fiscal year 2021. We also had an increase in acquisition costs of $0.8 million due to costs relating to the proposed merger with Great Outdoors Group, as well as increases in rent, depreciation, pre-opening, and other selling, general and administrative costs of $1.8 million, $1.3 million, $0.8 million, and $0.2 million, respectively, during the 13 weeks ended October 30, 2021 primarily related to the opening of seven new stores since October 31, 2020. As a percentage of net sales, selling, general, and administrative expenses increased to 24.9% of net sales in the third quarter of fiscal year 2021, compared to 23.9% of net sales in the third quarter of fiscal year 2020, due to the same reasons disclosed for the increase in selling, general, and administrative expenses.
Interest Expense. Interest expense decreased by $0.1 million, or 22.9%, to $0.4 million during the 13 weeks ended October 30, 2021 from $0.5 million for the comparable 13-week period of fiscal year 2020. Interest expense decreased primarily as a result of decreased borrowings on our term loan during the third quarter of fiscal year 2021 compared to the third quarter of fiscal year 2020.
Income Taxes. We recognized income tax expense of $7.4 million compared to an income tax expense of $9.5 million during the 13 weeks ended October 30, 2021 and October 31, 2020, respectively. Our effective tax rate for the 13 weeks ended October 30, 2021 and October 31, 2020 was 25.2% and 23.8%, respectively. Our effective tax rate will generally differ from the U.S. Federal statutory rate of 21.0%, due to state taxes, permanent items, and discrete items relating to stock award deductions.
Thirty-Nine Weeks Ended October 30, 2021 Compared to Thirty-Nine Weeks Ended October 31, 2020
Net Sales. Net sales increased by $76.2 million, or 7.5%, to $1,089.8 million during the 39 weeks ended October 30, 2021 compared to $1,013.6 million in the corresponding period of fiscal year 2020. Our net sales increased due to a variety of reasons including: the opening of seven new stores since October 31, 2020, increased participation in outdoor activities, increased demand through our e-commerce platform, and demand driven by the change in consumer behavior associated with the COVID-19 pandemic and social unrest, partially offset by lower demand during the second and third quarters of fiscal 2021 compared to the same periods in fiscal 2020 in certain categories as we anniversaried the demand